LeetCode 题解（165): Combinations

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题目：

Given two integers n and k, return all possible combinations ofk 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],]

题解：

Backtracing。

C++版：

class Solution {public:    vector<vector<int>> combine(int n, int k) {        vector<vector<int>> results;        if(n < k || n == 0 || k == 0)            return results;        vector<int> result;        for(int i = 1; i <= n; i++) {            result.push_back(i);            combines(results, result, i + 1, n, k - 1);            result.pop_back();        }        return results;    }        void combines(vector<vector<int>>& results, vector<int>& result, int i, int n, int k) {        if(k == 0) {            results.push_back(result);            return;        }        for(int j = i; j <= n; j++) {            result.push_back(j);            combines(results, result, j + 1, n, k - 1);            result.pop_back();        }    }};

Java版：

public class Solution {    public List<List<Integer>> combine(int n, int k) {        List<List<Integer>> results = new ArrayList<List<Integer>>();        if(n < k || n == 0 || k == 0)            return results;        List<Integer> result = new ArrayList<>();        for(int i = 1; i <= n; i++) {            result.add(i);            combines(results, result, i + 1, n, k - 1);            result.clear();        }        return results;    }        public void combines(List<List<Integer>> results, List<Integer> result, int i, int n, int k) {        if(k == 0) {            List<Integer> temp = new ArrayList<>();            temp.addAll(result);            results.add(temp);        }        for(int j = i; j <= n; j++) {            result.add(j);            combines(results, result, j + 1, n, k - 1);            result.remove(result.size() - 1);        }    }}

Python版：

import copyclass Solution:    # @param {integer} n    # @param {integer} k    # @return {integer[][]}    def combine(self, n, k):        if n < k or n == 0 or k == 0:            return []                    results, result = [], []        for i in range(1, n + 1):            result.append(i)            self.combines(results, result, i + 1, n, k - 1)            result.pop()        return results            def combines(self, results, result, i, n, k):        if k == 0:            results.append(copy.copy(result))            return        for j in range(i, n + 1):            result.append(j)            self.combines(results, result, j + 1, n, k - 1)            result.pop()

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