# First Missing Positive

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

**Example 1:**

```
Input: [1,2,0]
Output: 3
```

**Example 2:**

```
Input: [3,4,-1,1]
Output: 2
```

**Example 3:**

```
Input: [7,8,9,11,12]
Output: 1
```

**Note:**

Your algorithm should run in*O*(*n*) time and uses constant extra space.

## Solution

```
class Solution {
    public int firstMissingPositive(int[] nums) {

    }
}
```