LeetCode[398] Random Pick Index

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于 2016-10-17 14:08:36 发布

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文章标签： leetcode rand

本文链接：https://blog.youkuaiyun.com/wrsophia_977/article/details/52838333

本文介绍了一种在大规模数组中高效随机选取指定目标值索引的算法——水塘抽样。该算法适用于可能含有重复元素的大数组，并能在不消耗过多额外空间的情况下，确保每个符合条件的元素被选中的概率相同。

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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);

水塘抽样

class Solution {
public:
	Solution(vector<int> nums) {
		number = nums;
	}

	int pick(int target) {
		int alpha = 1;
		int resIndex;
		for (int i = 0; i < number.size(); ++i) {
			if (number[i] == target) {
				if (rand() % alpha == 0)
					resIndex = i;
				++alpha;
			}
		}
		return resIndex;
	}
private:
	vector<int> number;
};


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