77. Combinations (JAVA)
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], ]
带回溯的递归。(带回溯没法用循环实现)
class Solution { public List<List<Integer>> combine(int n, int k) { result = new ArrayList<>(); List<Integer> ans = new ArrayList<Integer>(); dfs(ans,n,k,1); return result; } void dfs(List<Integer> ans, int n, int k, int depth){ if(ans.size()==k) { List<Integer> new_ans = new ArrayList<Integer>(ans); result.add(new_ans); return; } if(depth>n){ return; } ans.add(depth); dfs(ans,n,k,depth+1); ans.remove(ans.size()-1); dfs(ans,n,k,depth+1); } private List<List<Integer>> result; }