LeetCode First Missing Positive

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.

Tips:

Try to put each element of the array in their right position. At most n times swap.

Solution:

class Solution {
public:
int firstMissingPositive(int A[], int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
for(int i=0;i<n;++i){
while(A[i]!=i+1){
if(A[i]>=1&&A[i]<=n&&A[A[i]-1]!=A[i])
swap(A[i],A[A[i]-1]);
else{
break;
}
}
}
for(int i=0;i<n;++i){
if(A[i]!=i+1)
return i+1;
}
return n+1;
}
};