Big O

function factorial(n) {
  if (n === 1) return 1;
  return n * factorial(n - 1);
}

Recursive Factorial

Visual Whiteboard of Rec Fac.

function fibonacci(n) {
  if (n === 1 || n === 2) return 1;
  return fib(n - 1) + fib(n - 2);
}

Recursive Fibonacci

Visual Whiteboard of Rec Fib.

Big O Notation

• Big O helps gives us a precise vocabulary to talk about how our code performs.

  • Useful for discussing trade-offs between different approaches.
  • Helps us debug things easier.
  • It also comes up a lot in interviews!

• An Example: Comparing two functions that calculate the sum of all numbers from 1 up to n.
  
  

function addUpTo(n) {
  let total = 0;
  for (let i = 0; i <= n; i++) {
    total += i;
  }
  return total;
}

Number of operations will grow with n.
Would be O(n) or Linear Time.

function addUpTo(n) {
  return (n * (n + 1)) / 2;
}

Has three simple operations: 1 Multiplication 1 Addition 1 Division. (Regardless of n)
Would be O(1) or Constant Time.

• First we need to consider what makes one implementation better than the other?

  • Faster? (Time Complexity);
  • Less Memory Intensive (Space Complexity);
  • More Readable

• How can we measure speed?

  • Timers? (Doesn't work well - not reliable or precise)
  • Instead we should count the number of simple operations.

• Big O Notation is a way to formalize fuzzy counting.

• An algorithm is O(f(n)) if the number of simple operations the computer has to do is eventually less than a constant f(n) times, as n increases.

  • f(n) = n (Linear)
  • f(n) = n^2 (Quadratic)
  • f(n) = 1 (Constant)
  • f(n) could be anything!

function countUpAndDown(n) {
  console.log('going up!');
  for (let i = 0; i < n; i++) {
    console.log(i);
  }
  console.log('at the top, going down!');
  for (let j = n - 1; j >=0; j--) {
    console.log(j);
  }
  console.log('Back down, bye!);
}

Both loops are O(n) but since we just want the big picture, this entire function would be O(n);

function printAllPairs(n) {
  for (var i = 0; i < n; i++) {
    for (var j = 0; j < n; j++) {
      console.log(i, j);
    }
  }
}

Nested loops are never a good thing when trying to write fast code.
O(n^2) or Quadratic Time.

• Constants don't matter in big O & Smaller Terms don't matter

  • O(2n) is just O(n) Linear
  • O(500) is just O(1) Constnat
  • O(13n^2) is just O(n^2) Quadratic
  • O(n + 10) is just O(n) Linear
  • O(1000n + 50) is just O(n) Linear
  • O(n^2 + 5n + 8) is just O(n^2) Quadratic

• Big O Shorthands

  • Arithmetic Operations are Constant
  • Variable assignment is constant
  • Accessing elements in an array (by index) or by object (by key) is constant.
  • In a loop, the complexity is the length of the loop times the complexity of whatever is inside of the loop.

• Additional Examples
  
  

function logAtLeast5(n) {
  for (var i = 1; i <= Math.max(5, n); i++) {
    console.log(i);
  }
}

O(n) Linear Time

function logAtMost5(n) {
  for (var i = 1; i <= Math.min(5, n); i++) {
    console.log(i);
  }
}

O(1) Constant Time.

A Guide to Big-O Notation

Curating Complexity: A Guide to Big-O Notation

• Why is looking at runtime not a reliable method of calculating time complexity?

  • Not all computers are made equal( some may be stronger and therefore boost our runtime speed )
  • How many background processes ran concurrently with our program that was being tested?
  • We also need to ask if our code remains performant if we increase the size of the input.

• The real question we need to answering is: How does our performance scale?.

Big O Notation

• Big O Notation is a tool for describing the efficiency of algorithms with respect to the size of the input arguments.
• Since we use mathematical functions in Big-O, there are a few big picture ideas that we'll want to keep in mind:
  • The function should be defined by the size of the input.
  • Smaller Big O is better (lower time complexity)
  • Big O is used to describe the worst case scenario.
  • Big O is simplified to show only its most dominant mathematical term.

Simplifying Math Terms

• We can use the following rules to simplify the our Big O functions:

  • Simplify Products : If the function is a product of many terms, we drop the terms that don't depend on n.
  • Simplify Sums : If the function is a sum of many terms, we drop the non-dominant terms.

• n : size of the input

• T(f) : unsimplified math function

• O(f) : simplified math function.

Simplifying a Product
| Unsimplified | Big-O Simplified |
| ------------ | ---------------- |
| T(5 _ n^2) | O(n^2) Quadratic |
| T(100000 _ n) | O(n) Linear |
| T( n / 12) | O (n) Linear |
| T( 42 _ n _ log(n)) | O(nlog(n)) Log Linear |
| T(12) | O(1) Constant |

Simplifying a Sum
| Unsimplified | Big-O Simplified |
| ------------ | ---------------- |
| T( n3 + n2 + n ) | O(n^3) |
| T( log(n) + 2n )| O(2^n) Exponential |
| T( n + log(n) ) | O(n) Linear |
| T( n! + 10n ) | O(n!) Polynomial |

Putting it all together

• First we apply the product rule to drop all constants.
• Then we apply the sum rule to select the single most dominant term.

Complexity Classes

Common Complexity Classes

There are 7 major classes in Time Complexity

• O(1) Constant

  • The algorithm takes roughly the same number of steps for any input size.

  // O(1)
  function constant1(n) {
    return n * 2 + 1;
  }

  // O(1)
  function constant2(n) {
    for (let i = 1; i <= 100; i++) {
      console.log(i);
    }
  }

• O(log(n)) Logarithmic

  • In most cases our hidden base of Logarithmic time is 2, log complexity algo's will typically display 'halving' the size of the input (like binary search!)

  // O(log(n))
  function logarithmic1(n) {
    if (n <= 1) return;
    logarithmic1(n / 2);
  }

  // O(log(n))
  function logarithmic2(n) {
    let i = n;
    while (i > 1) {
      i /= 2;
    }
  }

• O(n) Linear

  • Linear algo's will access each item of the input "once".

  // O(n)
  function linear1(n) {
    for (let i = 1; i <= n; i++) {
      console.log(i);
    }
  }

  // O(n), where n is the length of the array
  function linear2(array) {
    for (let i = 0; i < array.length; i++) {
      console.log(i);
    }
  }

  // O(n)
  function linear3(n) {
    if (n === 1) return;
    linear3(n - 1);
  }

• O(nlog(n)) Log Linear Time
  • Combination of linear and logarithmic behavior, we will see features from both classes.
  • Algo's that are log-linear will use both recursion AND iteration.

// O(n * log(n))
function loglinear(n) {
  if (n <= 1) return;

  for (let i = 1; i <= n; i++) {
    console.log(i);
  }

  loglinear(n / 2);
  loglinear(n / 2);
}

• O(nc) Polynomial
  • C is a fixed constant.

// O(n^2)
function quadratic(n) {
  for (let i = 1; i <= n; i++) {
    for (let j = 1; j <= n; j++) {}
  }
}

// O(n^3)
function cubic(n) {
  for (let i = 1; i <= n; i++) {
    for (let j = 1; j <= n; j++) {
      for (let k = 1; k <= n; k++) {}
    }
  }
}

• Example of Quadratic and Cubic runtime.

• O(c^n) Exponential

  • C is now the number of recursive calls made in each stack frame.
  • Algo's with exponential time are VERY SLOW.

// O(2^n)
function exponential2n(n) {
  if (n === 1) return;
  exponential_2n(n - 1);
  exponential_2n(n - 1);
}

// O(3^n)
function exponential3n(n) {
  if (n === 0) return;
  exponential_3n(n - 1);
  exponential_3n(n - 1);
  exponential_3n(n - 1);
}

• O(n!) Factorial
  • The largest/ worst complexity (minus DTIME which is n^n);

Memoization

• Memoization : a design pattern used to reduce the overall number of calculations that can occur in algorithms that use recursive strategies to solve.
  • MZ stores the results of the sub-problems in some other data structure, so that we can avoid duplicate calculations and only 'solve' each problem once.
  • Two features that comprise memoization:
  • 1. FUNCTION MUST BE RECURSIVE.
  • 2. Our additional DS is usually an object (we refer to it as our memo!)

Memoizing Factorial

let memo = {};

function factorial(n) {
  // if this function has calculated factorial(n) previously,
  // fetch the stored result in memo
  if (n in memo) return memo[n];
  if (n === 1) return 1;

  // otherwise, it havs not calculated factorial(n) previously,
  // so calculate it now, but store the result in case it is
  // needed again in the future
  memo[n] = n * factorial(n - 1);
  return memo[n];
}

factorial(6); // => 720, requires 6 calls
factorial(6); // => 720, requires 1 call
factorial(5); // => 120, requires 1 call
factorial(7); // => 5040, requires 2 calls

memo; // => { '2': 2, '3': 6, '4': 24, '5': 120, '6': 720, '7': 5040 }

• Our memo object is mapping out our arguments of factorial to it's return value.
  • Keep in mind we didn't improve the speed of our algo.

Memoizing Fibonacci

• Our time complexity for fibonacci goes from O(2^n) to O(n) after applying memoization.

The Memoization Formula

Rules

1. Write the unoptimized brute force recursion (make sure it works);
2. Add memo object as an additional arugmnt .
3. Add a base case condition that returns the stored value if the function's argument is in the memo.
4. Before returning the result of the recursive case, store it in the memo as a value and make the function's argument it's key.

Things to remember

1. When solving DP problems with Memoization, it is helpful to draw out the visual tree first.
2. When you notice duplicate sub-tree's that means we can memoize.

**

function fastFib(n, memo = {}) {
  if (n in memo) return memo[n];
  if (n === 1 || n === 2) return 1;

  memo[n] = fastFib(n - 1, memo) + fastFib(n - 2, memo);
  return memo[n];
}

fastFib(6); // => 8
fastFib(50); // => 12586269025

Tabulation

• Tabulation Strategy
  • Use When:
  • The function is iterative and not recursive.
  • The accompanying DS is usually an array.

function fib(n) {
  let mostRecentCalcs = [0, 1];

  if (n === 0) return mostRecentCalcs[0];

  for (let i = 2; i <= n; i++) {
    const [secondLast, last] = mostRecentCalcs;
    mostRecentCalcs = [last, secondLast + last];
  }

  return mostRecentCalcs[1];
}

• Steps for tabulation
  • Create a table array based off the size of the input.
  • Initialize some values in the table to 'answer' the trivially small subproblem.
  • Iterate through the array and fill in the remaining entries.
  • Your final answer is usually the last entry in the table.

Class Examples

Example of Linear Search

function search(array, term) {
  for (let i = 0; i < array.length; i++) {
    if (array[i] === term) {
      return i;
    }
  }
  return -1;
}

• Worst Case Scenario: The term does not even exist in the array.
• Meaning: If it doesn't exist then our for loop would run until the end therefore making our time complexity O(n).

Example of Binary Search

function binarySearch(arr, x, start, end) {
  if (start > end) return false;

  let mid = Math.floor((start + end) / 2);
  if (arr[mid] === x) return true;

  if (arr[mid] > x) {
    return binarySearch(arr, x, start, mid - 1);
  } else {
    return binarySearch(arr, x, mid + 1, end);
  }
}

• Must be conducted on a sorted array.
• Binary search is logarithmic time, not exponential b/c n is cut down by two, not growing.
• Binary Search is part of Divide and Conquer.

Example of Merge Sort

function merge(leftArray, rightArray) {
  const sorted = [];
  while (letArray.length > 0 && rightArray.length > 0) {
    const leftItem = leftArray[0];
    const rightItem = rightArray[0];

    if (leftItem > rightItem) {
      sorted.push(rightItem);
      rightArray.shift();
    } else {
      sorted.push(leftItem);
      leftArray.shift();
    }
  }

  while (leftArray.length !== 0) {
    const value = leftArray.shift();
    sorted.push(value);
  }

  while (rightArray.length !== 0) {
    const value = rightArray.shift();
    sorted.push(value);
  }

  return sorted;
}

function mergeSort(array) {
  const length = array.length;
  if (length === 1) {
    return array;
  }

  const middleIndex = Math.ceil(length / 2);
  const leftArray = array.slice(0, middleIndex);
  const rightArray = array.slice(middleIndex, length);

  leftArray = mergeSort(leftArray);
  rightArray = mergeSort(rightArray);

  return merge(leftArray, rightArray);
}

Example of Bubble Sort

function bubbleSort(items) {
  var length = items.length;
  for (var i = 0; i < length; i++) {
    for (var j = 0; j < length - i - 1; j++) {
      if (items[j] > items[j + 1]) {
        var tmp = items[j];
        items[j] = items[j + 1];
        items[j + 1] = tmp;
      }
    }
  }
}

• Worst Case & Best Case are always the same because it makes nested loops.
• Double for loops are polynomial time complexity or more specifically in this case Quadratic big O O(n^2);