398. Random Pick Index

Problem:

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);

思路：

Solution (C++):

vector<int> _nums;
Solution(vector<int>& nums) {
    _nums = nums;
}

int pick(int target) {
    int count = 0, res = -1;
    for (int i = 0; i < _nums.size(); ++i) {
        if (_nums[i] != target)  continue;
        if (count == 0)  { res = i; ++count; }
        else {
            ++count;
            if (rand()%count == 0)  res = i;
        }
    }
    return res;
}

性能：

Runtime: 96 ms  Memory Usage: 16.4 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB