leetcode12: 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.

 

 incomplete.

 

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