LeetCode 75 Combinations

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],
]

分析：

看到返回所有，想到回溯，那么就是DFS了。

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        
        List<List<Integer>> results = new ArrayList<List<Integer>>();
        if(k<0 || k>n) return results;
        List<Integer> result = new ArrayList<Integer>();
        dfs(results, result, n, k, 1);
        return results;
    }
    
    public void dfs(List<List<Integer>> results, List<Integer> result, int n, int k, int start){
        if(result.size()==k){
            results.add(new ArrayList<Integer>(result));
            return;
        }
        for(int i=start; i<=n; i++){
            //加入结果
            result.add(i);
            //往后搜索
            dfs(results, result, n, k, i+1);
            //恢复现场
            result.remove(result.size()-1);
        }
    }
}