Implementing the randomized set class with O(1) operation

Question

This is related to the leetcode - question here, which asks:

Implement the RandomizedSet class:

• RandomizedSet() Initializes the RandomizedSet object.
• bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise.
• bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise.
• int getRandom() Returns a random element from the current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.

You must implement the functions of the class such that each function works in average O(1) time complexity.

I am able to pass some of the test cases but it fails one of the test cases (where in some places, my program returns undefined). What is it that I am doing wrong here? I am unable to find my mistake.

var RandomizedSet = function() {
    this.map = new Map();
    this.vector = [];
};

/** 
* @param {number} val
* @return {boolean}
*/
RandomizedSet.prototype.insert = function(val) {
    if(this.map.has(val)) return false;
    else {
        let position = 0;
        if(this.vector.length > 0)
            position = this.vector.length-1;
        
        this.map.set(val, position);
        this.vector.push(val);
        return true;
    }
};

/** 
* @param {number} val
* @return {boolean}
*/
RandomizedSet.prototype.remove = function(val) {
    if(this.map.has(val)) {
        const index = this.map.get(val);
        const lastVal = this.vector[this.vector.length-1];
        this.vector[index] = lastVal;
        this.vector.pop();
        this.map.delete(val);
        return true;
    }else {
        return false;
    }
};

/**
* @return {number}
*/
RandomizedSet.prototype.getRandom = function() {
    const randIndex = Math.floor(Math.random() * this.vector.length);
    return this.vector[randIndex];
};

/** 
* Your RandomizedSet object will be instantiated and called as such:
* var obj = new RandomizedSet()
* var param_1 = obj.insert(val)
* var param_2 = obj.remove(val)
* var param_3 = obj.getRandom()
*/

Answer 1

When moving the last element into the place of the removed element, you forgot to update the moved element's index in the map:

RandomizedSet.prototype.remove = function(val) {
    if(this.map.has(val)) {
        const index = this.map.get(val);
        const lastVal = this.vector[this.vector.length-1];
        this.vector[index] = lastVal;
        this.vector.pop();
        this.map.set(lastVal, index); // ADDED
        this.map.delete(val);
        return true;
    }else {
        return false;
    }
};

Be aware of the case index == this.vector.length-1; the order of operations shown above will handle this case correctly without additional code.