Random Pick Index

Problem statement

Given an integer array nums with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the array nums.
• int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i's, then each index should have an equal probability of returning.

Example 1:

Input["Solution", "pick", "pick", "pick"][[[1, 2, 3, 3, 3]], [3], [1], [3]]Output[null, 4, 0, 2]ExplanationSolution solution = new Solution([1, 2, 3, 3, 3]);solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

Constraints:

• 1 <= nums.length <= 2 * 104
• -231 <= nums[i] <= 231 - 1
• target is an integer from nums.
• At most 104 calls will be made to pick.

My solution

/**
 * @param {number[]} nums
 */
var Solution = function(nums) {
    this.map = new Map();
    
    for (let i = 0; i < nums.length; i++) {
        const num = nums[i]
        if (!this.map.has(num)) {
            this.map.set(num, [])
        }
        this.map.get(num).push(i)
    }
};

/** 
 * @param {number} target
 * @return {number}
 */
Solution.prototype.pick = function(target) {
    const idxs = this.map.get(target);
    if (idxs.length === 1) {
        return idxs[0]
    } else {
        const lastValue = idxs.shift()
        idxs.push(lastValue)
        return lastValue
    }
};

/** 
 * Your Solution object will be instantiated and called as such:
 * var obj = new Solution(nums)
 * var param_1 = obj.pick(target)
 */