398. Random Pick Index

398. Random Pick Index

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);

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很直接的Reservoir sampling, 看总结

class Solution {

    private int[] nums;
    private Random rand;
    public Solution(int[] nums) {
        this.nums = nums;
        rand = new Random();
    }
    
    public int pick(int target) {
        int size = 0;
        int index = -1;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                size++;
                if (rand.nextInt(size) == 0) {
                    index = i;
                }
            }
        }
        
        return index;
    }
}

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