题目地址：

https://www.lintcode.com/problem/longest-prefix-of-array/description

给定一个数组$A$，再给定两个数$x$和$y$，求最长的$A$前缀$A[0:i]$使得$x$和$y$在其中的出现次数相等，且出现次数都大于$0$。

用两个变量记录前缀里两个数字出现次数即可。代码如下：

public class Solution { /** * @param X: a integer * @param Y: a integer * @param nums: a list of integer * @return: return the maximum index of largest prefix */ public int LongestPrefix(int X, int Y, int[] nums) { // write your code here int countX = 0, countY = 0; int res = -1; for (int i = 0; i < nums.length; i++) { if (nums[i] == X) { countX++; } if (nums[i] == Y) { countY++; } if (countX == countY && countX > 0) { res = Math.max(res, i); } } return res; } }

时间复杂度$O(n)$，空间$O(1)$。