LeetCode - 41. First Missing Positive

41. First Missing Positive #

Problem's Link
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Mean: 

给你一组整数，找出第一个空缺的正整数.

要求：时间O(n),空间O(n).

analyse:

这题时间O(n)想了半天没想到，用O(n*logn)过的.

然后看了discuss，想法非常巧妙，自愧不如.

Time complexity: O(N)

 

view code

/**
* -----------------------------------------------------------------
* Copyright (c) 2016 crazyacking.All rights reserved.
* -----------------------------------------------------------------
*       Author: crazyacking
*       Date  : 2016-03-05-18.56
*/
#include <queue>
#include <cstdio>
#include <set>
#include <string>
#include <stack>
#include <cmath>
#include <climits>
#include <map>
#include <cstdlib>
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
using namespace std;
typedef long long(LL);
typedef unsigned long long(ULL);
const double eps(1e-8);

class Solution
{
public:
   vector<vector<int>> combinationSum2(vector<int>& candidates, int target)
   {
       sort(candidates.begin(),candidates.end());
       vector<vector<int>> res;
       vector<int> combination;
       combinationSum(candidates,res,combination,target,0);
       return res;
   }
private:
   void combinationSum(vector<int>& candidates,vector<vector<int>> &res,vector<int>& combination,int target,int begin)
   {
       if(!target)
       {
           res.push_back(combination);
           return;
       }
       for(int i=begin;target>=candidates[i] && i<candidates.size() ;++i)
       {
           if(i==begin || candidates[i]!=candidates[i-1])
           {
               combination.push_back(candidates[i]);
               combinationSum(candidates,res,combination,target-candidates[i],i+1);
               combination.pop_back();
           }
       }
       return;
   }
};


int main()
{
   freopen("H:\\Code_Fantasy\\in.txt","r",stdin);
   int n,target;
   while(cin>>n>>target)
   {
       cout<<n<<" "<<target<<endl;
       vector<int> ve;
       for(int i=0; i<n; ++i)
       {
           int tmp;
           cin>>tmp;
           ve.push_back(tmp);
       }
       Solution solution;
       vector<vector<int>> ans=solution.combinationSum2(ve,target);
       for(auto p1:ans)
       {
           for(auto p2:p1)
           {
               cout<<p2<<" ";
           }
           cout<<endl;
       }
   }
   return 0;
}
/*

*/

O(n)时间，O(1)空间的代码：

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