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.

采用hash的思路，将A[i]的当做索引，标记对应的A[A[i]-1]为-，  最后遍历，为正的则为缺失的

 

 

    int firstMissingPositive(int A[], int n) {
        for (int i=0; i<n; i++)
        {
            if (A[i] <= 0)
                A[i] = n+2;
        }
        
        for (int i=0; i<n; i++)
        {
            int nTmp = abs(A[i]);
            if (nTmp<1 || nTmp>n)
                continue;
            
            if (A[nTmp-1] > 0)
                A[nTmp-1] *= -1;
        }
        
        for (int i=0; i<n; i++)
        {
            if (A[i] > 0)
                return i+1;
        }
        
        return n+1;
    }