Missing number

Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.

For example,
Given nums = [0, 1, 3] return 2.

Note:

Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?

方法一：

求和相减得到的就是消失的。

[cpp]  view plain  copy

1. class Solution {  
2. public:  
3.     int missingNumber(vector<int>& nums) {  
4.         int len = nums.size();  
5.         int sum = (1 + len)*len / 2;  
6.         int sum2 = 0;  
7.         for (int n : nums){  
8.             sum2 += n;  
9.         }  
10.         return sum - sum2;  
11.     }  
12. };  

方法二：异或

跟 

137. Single Number II   

260. Single Number III 类似

[cpp]  view plain  copy

1. class Solution {  
2. public:  
3.     int missingNumber(vector<int>& nums) {  
4.         int res=0;  
5.         for(int i=0;i<nums.size();i++){  
6.             res^=(i+1)^nums[i];  
7.         }  
8.         return res;  
9.     }  
10. };