leetcode-First Missing Positive

Posted 2020-09-22 jiu__

https://leetcode.com/problems/first-missing-positive/#/description

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

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

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

 

题解：这道题目比较简单，需要注意的是几个陷阱：数组为空，数组里数据重复。

比较有技巧的地方是O(1)space，因为考虑到数据重复，此处只能排序， 这就要求重用nums数组空间。

引用网上一位网友的题解：

Put each number in its right place.

For example:

When we find 5, then swap it with A[4].

At last, the first place where its number is not right, return the place + 1.

程序：

 1 class Solution {
 2 public:
 3     int firstMissingPositive(vector<int>& nums) {
 4         for (int i = 0; i < nums.size(); i++)
 5         {
 6             while (nums[i] > 0 && nums[i] <= nums.size() && nums[i] != nums[nums[i] -1])
 7             {
 8                 swap(nums[i], nums[nums[i] -1]);
 9             }
10         }
11         for (int i = 0; i < nums.size(); i++)
12         {
13             if (nums[i] != i + 1)
14             {
15                 return i + 1;
16             }
17         }
18         return nums.size() + 1;
19     }
20 };