Leetcode: First Missing Positive

Problem:

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.

Solution:

Put all the number in A into a HashSet,

Then for integers from 1 to A.length, check if they have been contained in the set.

一次AC啦，开胸。

Code:

public class Solution {
public int firstMissingPositive(int[] A) {
HashSet<Integer> set = new HashSet<Integer>();
for (int i = 0; i < A.length; i++) {
set.add(A[i]);
}
for (int i = 1; i <= A.length; i++) {
if (!set.contains(i)) return i;
}
return A.length + 1;
}
}