41. 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.

class Solution {
    public void swap(int[] nums, int a, int b){
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        int i = 0;
        while (i < n) {
            if (nums[i] > 0 && nums[i] <= n) {
                if (i != nums[i] - 1  
                    && nums[i] != nums[nums[i] - 1]) {
                    swap(nums, i, nums[i] - 1);
                } else {
                    i++;
                }
            } else {
                i++;
            }
        }
        
        int ans = -1;
        for (i = 0; i < n; ++i) {
            if (nums[i]  != i + 1) {
                ans = i + 1;
                
                break;
            }
        }
        
        return ans == -1 ? n + 1 : ans;
    }
}