leetcode 77. Combinations-排列|递归|非递归|Java|Python

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原题链接：77. Combinations

【思路-Java、Python】递归实现

采用回溯算法。这是一道 NP 难问题，时间复杂度没办法提高，用一个循环递归处理子问题，问题的终止条件是每个组合中的元素个数达到 k 个：

public class Solution {    public List<List<Integer>> combine(int n, int k) {        List<List<Integer>> res = new ArrayList<List<Integer>>();        List<Integer> temp = new ArrayList<Integer>();        dfs(res, temp, n, k, 1);        return res;    }    private void dfs(List<List<Integer>> res, List<Integer> temp, int n, int k, int m) {        if(k == 0) {            res.add(new ArrayList<Integer>(temp));            return;        }        for(int i=m; i<=n; i++) {            temp.add(i);            dfs(res, temp, n, k-1, i+1);            temp.remove(temp.size()-1);        }    }}

26 / 26 test cases passed. Runtime: 3 ms Your runtime beats 56.70% of javasubmissions.

class Solution(object):    def combine(self, n, k):        """        :type n: int        :type k: int        :rtype: List[List[int]]        """        res = []        self.rec(res, 0, n, k, [])        return res    def rec(self, res, i, n, k, temp) :        if k == 0 :            res.append(temp)            return        for j in range(i+1, n+1) :            self.rec(res, j, n, k-1, temp+[j])

26 / 26 test cases passed. Runtime: 88 ms Your runtime beats 32.21% of pythonsubmissions.

【思路2-Python】非递归实现

class Solution(object):    def combine(self, NN, K):        """        :type n: int        :type k: int        :rtype: List[List[int]]        """        result = [[[]]]        for n in range(1,NN+1):            newRes=[[[]]]            for k in range(1,n):                newRes.append(result[k] + [i + [n] for i in result[k-1]])            newRes.append([result[n-1][0] + [n]])            result = newRes        return result[K]

26 / 26 test cases passed. Runtime: 88 ms Your runtime beats 32.21% of pythonsubmissions.

更优解法可参考：优化解法

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