First Missing Positive

Problem link

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

Example:

Input: [1,2,0]

Output: 3

Discussion :

class Solution {
    public int firstMissingPositive(int[] A) {
        int n = A.length;
        if (n == 0) return 1;
        for (int i = 0; i < n; ) {
            if (A[i] > 0 && A[i] <= n && A[A[i]-1] != A[i]){
                int a = A[i];
                A[i] = A[a - 1];
                A[a - 1] = a;
            }
            else ++i;
        }
    
        for (int i = 0; i < n; ++i){
            if (A[i] != i + 1) 
                return i + 1;
        }
        return n+1;
    }
}