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.

 1 class Solution {
 2     public void swap(int[] nums, int a, int b){
 3         int temp = nums[a];
 4         nums[a] = nums[b];
 5         nums[b] = temp;
 6     }
 7     public int firstMissingPositive(int[] nums) {
 8         int n = nums.length;
 9         int i = 0;
10         while (i < n) {
11             if (nums[i] > 0 && nums[i] <= n) {
12                 if (i != nums[i] - 1  
13                     && nums[i] != nums[nums[i] - 1]) {
14                     swap(nums, i, nums[i] - 1);
15                 } else {
16                     i++;
17                 }
18             } else {
19                 i++;
20             }
21         }
22         
23         int ans = -1;
24         for (i = 0; i < n; ++i) {
25             if (nums[i]  != i + 1) {
26                 ans = i + 1;
27                 
28                 break;
29             }
30         }
31         
32         return ans == -1 ? n + 1 : ans;
33     }
34 }