LeetCode:Combinations

Combinations

Total Accepted: 75841 Total Submissions: 219221 Difficulty: Medium

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

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 (M) Combination Sum (M) Permutations

思路：

例：n = 4, k = 3;有combs,comb数组

组合情况有：

1）1 2 3

2）1 2 4

3）1 3 4

4）2 3 4

生成过程：

comb=[], k=3,初始状态;

comb=[1], k=2,加入1;

comb=[1,2], k=1,加入2;

comb=[1,2,3], k=0;加入3,结果添加到combs;

comb=[1,2], k=1;去掉3

comb=[1,2,4], k=0;加入3,结果添加到combs;

comb=[1,2], k=1,去掉4;

comb=[1], k=2,这时由于已经遍历到4(==n),要再去掉2;

comb=[1,3], k=1,加入3;

comb=[1,3,4], k=0,加入4,结果添加到combs;

comb=[1,3], k=1,去掉4;

comb=[1], k=2,这时由于已经遍历到n(==4),要再去掉3;

comb=[1,4], k=1,加入4;已经到n;

comb=[1], k=2,去掉4;

comb=[], k=3,去掉1;

comb=[2], k=2,加入2;

comb=[2,3], k=1,加入3;

comb=[2,3,4], k=0,加入4,结果添加到combs;

c++ code:

class Solution {public:    vector<vector<int>> combine(int n, int k) {        vector<vector<int>> combs;        vector<int> comb;        combine(combs,comb,1,n,k);        return combs;    }    void combine(vector<vector<int>> &combs, vector<int> &comb,int begin,int n,int k) {                if(k==0){            combs.push_back(comb);            return;        }                for(int i=begin;i<=n;i++) {            comb.push_back(i);            combine(combs,comb,i+1,n,k-1);            comb.pop_back();        }    }};