LeetCode-in-Java

77. Combinations

Medium

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

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2

Output: [ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]

Example 2:

Input: n = 1, k = 1

Output: [[1]]

Constraints:

• 1 <= n <= 20
• 1 <= k <= n

Solution

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;

@SuppressWarnings("java:S1149")
public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> ans = new ArrayList<>();
        // Boundary case
        if (n > 20 || k < 1 || k > n) {
            return ans;
        }
        backtrack(ans, n, k, 1, new ArrayDeque<>());
        return ans;
    }

    private void backtrack(
            List<List<Integer>> ans, int n, int k, int s, ArrayDeque<Integer> stack) {
        // Base case
        // If k becomes 0
        if (k == 0) {
            ans.add(new ArrayList<>(stack));
            return;
        }
        // Start with s till n-k+1
        for (int i = s; i <= (n - k) + 1; i++) {
            stack.push(i);
            // Update start for recursion and decrease k by 1
            backtrack(ans, n, k - 1, i + 1, stack);
            stack.pop();
        }
    }
}

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