77. 组合

class Solution {
    LinkedList<Integer> path = new LinkedList<>();
    List<List<Integer>> res = new LinkedList<>();
    public List<List<Integer>> combine(int n, int k) {
        backtracking(n, k, 1);
        return res;
    }
    private void backtracking(int n, int k, int startIndex){
        if(path.size() == k){
            res.add(new LinkedList<>(path));
            return;
        }

        for(int i = startIndex; i <= n - (k - path.size()) + 1; i++){
            path.add(i);
            backtracking(n, k, i + 1);
            path.removeLast();
        }
    }
}