540. Single Element in a Sorted Array

You are given a sorted array consisting of only integers where every element appears exactly twice, except for one element which appears exactly once. Find this single element that appears only once.

Example 1:

Input: [1,1,2,3,3,4,4,8,8]
Output: 2

Example 2:

Input: [3,3,7,7,10,11,11]
Output: 10

class Solution {
    public int singleNonDuplicate(int[] nums) {
        int res = 0;
        for(int i: nums) res = res^i;
        return res;
    }
}

这题不涉及O（logN）还是很愉快的，一波异或带走，和single number一样

假如要求O（logN），说明要二分法

class Solution {
    public int singleNonDuplicate(int[] nums) {
        int left = 0, right = nums.length - 1;
        while(left < right){
            int mid = right + (left-right)/2;
            if(mid % 2 == 1) mid--;
            if(nums[mid] == nums[mid + 1]) left += 2;
            else right = mid;
        }
        return nums[left];
    }
}

观察一下，mid和mid+1如果相等就说明在右边，否则一定在左边

比如1，1，2，2，3，4，4，先设定mid一定是偶数，才能比较mid和mid+1，因为left<right，所以mid+1一定存在。如果mid是奇数就要mid--。

 https://github.com/Cee/Leetcode/blob/master/540%20-%20Single%20Element%20in%20a%20Sorted%20Array.java

带个大佬的图比较清楚