leetcode -- 77. Combinations

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题目描述

题目难度:Medium
Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

• Example:

Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

AC代码

class Solution {public List<List<Integer>> combine(int n, int k) {List<List<Integer>> resList = new ArrayList<>();if(n <= 0 || k <= 0 || n < k) return resList;backtracing(n, k, 1, new ArrayList<Integer>(), resList);return resList;}private void backtracing(int n, int k, int start, List<Integer> tmpList, List<List<Integer>> resList){if(k == tmpList.size()) {resList.add(new ArrayList<>(tmpList));return;}for(int i = start;i <= n;i++){tmpList.add(i);backtracing(n, k, i + 1, tmpList, resList);tmpList.remove(tmpList.size() - 1);}}
}

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