41. First Missing Positive

Given an unsorted integer array, find the first missing positive integer.

For example,

Given 

[1,2,0]

 return 

3

,

and 

[3,4,-1,1]

 return 

2

.

Your algorithm should run in O(n) time and uses constant space.

算法：将nums[i] 放置到正确的位置

class Solution {
public:
int firstMissingPositive(vector<int>& nums) {
int n = nums.size();
for(int i = 0; i < n; i++)
while(nums[i] > 0 && nums[i] <= n && nums[nums[i] - 1] != nums[i])
swap(nums[nums[i] - 1] , nums[i]);
for(int i = 0; i < n; i++)
if(nums[i] != i + 1) return i + 1;
return n + 1;
}
};