First Missing Positive.java

public class Solution {
    public int firstMissingPositive(int[] A) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(A.length==0) return 1; 
        for(int i=0; i<A.length; i++)
        {
            if(A[i]<=0) A[i]=A.length+1; 
        }
        for(int i=0; i<A.length; i++)
        {
            int num=Math.abs(A[i]);
            if(num>A.length || A[num-1]==-num) continue;
            if(A[num-1]==num)
            {
                A[num-1]=-num;  continue; 
            }
            int a=A[num-1]; 
            A[num-1]=-A[i]; 
            A[i]=a; 
            i--; 
        }
        for(int i=0; i<A.length; i++)
        {
            if(A[i]>0) return i+1; 
        }
        return A.length+1; 
    }
}