RandomPickIndex.cpp

// Source : https://leetcode.com/problems/random-pick-index/
// Author : Hao Chen
// Date   : 2016-11-04

/*************************************************************************************** 
 *
 * Given an array of integers 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.
 * 
 * Note:
 * The array size can be very large. Solution that uses too much extra space will not 
 * pass the judge.
 * 
 * Example:
 * 
 * int[] nums = new int[] {1,2,3,3,3};
 * Solution solution = new Solution(nums);
 * 
 * // pick(3) should return either index 2, 3, or 4 randomly. Each index should have 
 * equal probability of returning.
 * solution.pick(3);
 * 
 * // pick(1) should return 0. Since in the array only nums[0] is equal to 1.
 * solution.pick(1);
 ***************************************************************************************/

class Solution {
private:
    vector<int> nums;    
public:
    Solution(vector<int> nums) {
        srand(time(0));
        this->nums = nums;
    }
    
    int pick(int target) {
	// we just randomly pick a number from the array,
        // if the number is target just return the index.
        // otherwise, keep picking the number randomly.
        while(true) {
            int idx = rand() % nums.size();
            if ( target == nums[idx] ) {
                return idx;
            }
        }
    }
};

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