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

Follow up:

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

 

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        //遍历一遍，将所有的小于n的正数放到对应的位置
        int n = nums.size();
        for(int i=0;i<n;){
            if(nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i]-1]){
                swap(nums[i],nums[nums[i]-1]);
            }else{
                i++;
            }
        }
        //寻找第一个不符合要求的正整数
        for(int i=0;i<n;i++){
            if(nums[i] != i+1)
                return i+1;
        }
        return n+1;
        
    }
};