# 41. First Missing Positive (Hard)

https://leetcode.com/problems/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.

## Solutions

``````class Solution {
public int firstMissingPositive(int[] nums) {
if (nums == null || nums.length == 0) {
return 1;
}

// Make full use of the parameter array 'nums' since extra memory is not allowed

// The key point is to swap values to its corresponding slots, say nums[0] sets to value 1.
for (int i = 0; i < nums.length; i++) {
// Swap the number until they are in the right position.
while (nums[i] >= 1 && nums[i] <= nums.length && nums[i] != nums[nums[i] - 1]) {
int temp = nums[nums[i] - 1];
nums[nums[i] - 1] = nums[i];
nums[i] = temp;
}
}

// Next go through the manipulated array from left to right, the first slot i
// that the assigned value not equals to i+1 should be the answer.
for (int i = 0; i < nums.length; i++) {
if (nums[i] != i + 1) {
return i + 1;
}
}

return nums.length + 1;
}
}
``````