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;
    }
};