【Leetcode】398. Random Pick Index
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问题分析:
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);
题目的大意是说个给你一个数组,在给你数组中存在的一个数字,你需要随机给出一个该数字在数组中的位置。
分析:
题目说不能用太多的extra space,虽然本来就没想用。想到之前使用的一个蓄水池原理,可以在不知道链表总长度的情况下,等概率的选取链表中的元素。
呃,蓄水池原理各位自行百度吧~
算法概况:时间复杂度O(n),空间复杂度O(1)
贴上代码:
import java.util.Random;
public class Solution {
int[] nums;
int res;
Random r = new Random();
public Solution(int[] nums) {
this.nums = nums;
}
public int pick(int target) {
int count=1;
for(int i=0;i<nums.length;i++){
if(nums[i] == target){
if(r.nextInt(count)+1 == count){
res = i;
}
count++;
}
}
return res;
}
}
/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(nums);
* int param_1 = obj.pick(target);
*/
贴上结果:
速度还行吧
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