【CODE】First Missing Positive

41. First Missing Positive

Hard

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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.

• 思路借鉴：https://www.cnblogs.com/grandyang/p/4395963.html
• 还是使用交换的手法，将[1,n-1]之间的数字交换到他应该在的位置，然后再进行一次遍历，得出没有放置应有数字的位置：
• Runtime: 0 ms, faster than 100.00% of C++ online submissions for First Missing Positive.
• Memory Usage: 8.7 MB, less than 76.00% of C++ online submissions for First Missing Positive.
• Next challenges:
• Couples Holding Hands

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        int n=nums.size();
        if(n==0) return 1;
        for(int i=0;i<n;i++){
            if(nums[i]>0 && nums[i]<n && nums[nums[i]-1]!=nums[i]){
                swap(nums[i],nums[nums[i]-1]);
                i--;
            }
        }
        for(int i=0;i<n;i++){
            if(nums[i]!=i+1) return i+1;
        }
        return n+1;
    }
};