LeetCode刷题 Day24

LeetCode刷题 Day24

77. Combinations

Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: There are 4 choose 2 = 6 total combinations.
Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.

Example 2:

Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.

思路:

• 这里要弄清楚回溯树的level 和 for循环的关系， for循环是横向的，level是纵向的，引用一下代码随想录的图思路就比较清楚了

代码:

var combine = function(n, k) {
    let res = [];
    
    var helper = function(path, level) {
        if (path.length === k) {
            res.push([...path]);
            return;
        }
        
        for (let i = level; i <= n; i++) {
            path.push(i);
            helper(path, i + 1);
            path.pop();
        }
    }
    
    helper([], 1);
    return res;
};