leetcode講解--540. Single Element in a Sorted Array

題目

Given a sorted array consisting of only integers where every element appears twice except for one element which appears 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

Note: Your solution should run in O(log n) time and O(1) space.

題目地址

講解

二分查找，包含一個數的一邊個數必定是奇數個。

java代碼

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