41. First Missing Positive (JAVA)

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.

 

因为不能额外使用空间，所以不能用HashTable，利用原本的数组记录状态。将值为正整数的数字放到对应值-1的下标位置。

注意无限循环：比如[1,1]，所以nums[nums[i]-1]==nums[i]的情况要排除。

class Solution {
    public int firstMissingPositive(int[] nums) {
        int tmp;
        int i = 0;
        while(i < nums.length){
            if(nums[i] < nums.length && nums[i] > 0 && nums[i] != i+1 && nums[nums[i]-1]!=nums[i]){
                //put nums[i] at position of nums[i]-1
                tmp = nums[i];
                nums[i] = nums[tmp-1];
                nums[tmp-1] = tmp;
            }
            else{
                i++;
            }
        }
        
        for(i = 0; i < nums.length; i++){
            if(nums[i] != i+1) break;
        }
        return i+1;
    }
}