380. Insert Delete GetRandom O(1) [Leetcode][C++]

class RandomizedSet {
    // Optimal Solution Time O(1) & Auxiliary Space O(N)
private:
    vector<int> a; // array vector
    unordered_map<int,int> m; // Unordered Map does searching, insertion & deletion of element in O(1) time
public:
    /** Initialize your data structure here. */
    RandomizedSet() {
    // No need to initialise a & m as they are initialised automatically
    // to 0 as and when their container size increases.
    }

    /** Inserts a value to the array vector. Returns true if the array did not already contain the specified element. */
    bool insert(int val) {
        if(m.find(val)!=m.end())
            // If val is not already present in the map, find() function  
            // returns an iterator(m.end()) pointing to next memory location  
            // of the last element of the map. Otherwise, find() returns an iterator 
            // pointing to val which was already present in the map.  
            return false;
        else{
            a.push_back(val);  // insert val at the end of the array
            m[val]=a.size()-1; // m[key,value] stores the array element and 
                               // its index as key=array element & value=array element index
            return true;
        }
    }

    /** Removes a value from the array vector. Returns true if the array contained the specified element. */
    bool remove(int val) {
        if(m.find(val)==m.end()) // val not present in the array vector
            return false;
        else{
            // val present in the array vector
            // For example: a=[8,4,3,2], m={[8,0],[4,1],[3,2],[2,3]}, val=4, last=2
            // After a[m[val]]=a.back(); a=[8,2,3,2], m={[8,0],[4,1],[3,2],[2,3]}
            // After a.pop_back(); a=[8,2,3], m={[8,0],[4,1],[3,2],[2,3]}
            // After m[last]=m[val]; a=[8,2,3], m={[8,0],[4,1],[3,2],[2,1]}
            // After m.erase(val); a=[8,2,3], m={[8,0],[3,2],[2,1]}
            int last=a.back();  // back() fetches last element of the array vector
            a[m[val]]=a.back(); // m[val] locates the index of val in the array vector.
                                // Then we copy array last element value to the val location in the array
            a.pop_back();       // Delete the last element of the array 
            m[last]=m[val];     // In hashmap, assign index of val in array to the index of the last element   
            m.erase(val);       // Delete the val entry from map
            return true;
        }
    }

    /** Get a random element from the array vector */
    int getRandom() {
        // rand() function gives random value in the range of 0 to RAND_MAX(whose value is 32767). x%y gives 
        // remainder when x is divided by y and this remainder is in the range of 0 to y-1.
        // rand()%a.size() gives random value in the range of (0 to a.size()-1).
        // a[rand()%a.size()] will give random value of array in the range of a[0] to a[a.size()-1].
        return a[rand()%a.size()];
    }
};