leetcode-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.

class Solution {public:    int firstMissingPositive(int A[], int n) {        sort(A, A+n);        int i = 0;        int j = 1;        while((A[i] <= 0)&&(i < n)) i++;        if((i == n)||(A[i] != 1))return 1;        for(; i < n-1; i++)        {            if((A[i+1] == A[i])||(A[i+1] == A[i]+1))continue;            else break;        }        return A[i]+1;    }};