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],]
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题目

组给出两个整数n和k，返回从1......n中选出的k个数的组合。
样例
例如 n = 4 且 k = 2
返回的解为：
[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4]]

分析

典型的深度搜索回溯问题

代码

public class Solution {
    /**
     * @param n: Given the range of numbers
     * @param k: Given the numbers of combinations
     * @return: All the combinations of k numbers out of 1..n
     */
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        
        ArrayList<Integer> list = new ArrayList<>();
        
        if(n < k)
            return res;
        
        dfs(res, list, n, k, 1);
        
        return res;
    }

    private void dfs(ArrayList<ArrayList<Integer>> res, ArrayList<Integer> list, int n, int k, int start) {
        
        if(list.size() == k) {
            res.add(new ArrayList<>(list));
            return;
        }
        
        for(int i=start;i<=n;i++) {
            if(!list.contains(i))
                list.add(i);
            dfs(res, list, n, k, i+1);
            list.remove(list.size()-1);
        }
    }
}