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.

注意数组越界！

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
       if(0 == n || NULL == A)
    	   return 1;
	   for(int i = 0; i < n; ++i)
	   {
		   A[i] = A[i] < 0? 0: A[i];
	   }
	   for(int i = 0; i < n; ++i)
	   {
		   int idx = abs(A[i]) - 1;
		   if(idx >= 0 && idx < n)
		   {
			   if(A[idx] > 0)
				   A[idx] *= -1;
			   else if(0 == A[idx])
				   A[idx] = -1*(idx+1);
		   }
	   }

	   for(int i = 0; i < n; ++i)
		   if(A[i] >= 0)
			   return i+1;
	   return n+1;
    }
};