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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0077_combinations;

// #Medium #Backtracking #Algorithm_I_Day_11_Recursion_Backtracking
// #2023_08_11_Time_11_ms_(77.40%)_Space_93_MB_(5.21%)

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

/**
 * 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`
**/
@SuppressWarnings("java:S1149")
public class Solution {
    public List> combine(int n, int k) {
        List> 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> ans, int n, int k, int s, ArrayDeque 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();
        }
    }
}