[Leetcode] 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],
]

比较模板化的题目，典型的DFS问题，有好多种题目可以套用类似的模板。

    
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        List<Integer> current = new ArrayList<Integer>();
        if(n < k) return result;
        comb(n, 1, k, current, result);
        return result;
    }
    
    private void comb(int n, int beg, int k, List<Integer> current, List<List<Integer>> result) {
        if(k == 0) {
            result.add(new ArrayList<Integer>(current));
        }
        else {
            if(beg > n) return;
            for(int i = beg; i <= n; i++) {
                current.add(i);
                comb(n, i+1, k-1, current, result);
                current.remove(current.size() - 1);
            }
        }
    }