Lintcode: Combination

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

Have you met this question in a real interview? 

Yes

Example

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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Backtracking Array

Reference:

//thought

http://siddontang.gitbooks.io/leetcode-solution/content/backtracking/combination.html

//codes

http://bangbingsyb.blogspot.com/2014/11/leetcode-combinations.html

DFS  recursively  -- backtracking problem
Time: O(n!)   -- also worst case
Space: O(k)  ---- longest depth is k  ---- Worst case O(n)

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class Solution { public: /** * @param n: Given the range of numbers * @param k: Given the numbers of combinations * @return: All the combinations of k numbers out of 1..n */ vector<vector<int> > combine(int n, int k) { // write your code here vector<int> tempres; vector<vector<int> >res; DFS(1, n, k, tempres, res); return res; } void DFS(int start, int n, int k, vector<int> &tempres, vector<vector<int> > &res){ if(k == 0) { res.push_back(tempres); return; } for(int i = start; i <= n; i++){ tempres.push_back(i); DFS(i+1, n, k-1, tempres, res); tempres.pop_back(); } } };