LC 77. Combinations (Rust BT)

Author: raul-sauco

README.md

@@ -80,6 +80,7 @@ Solutions to LeetCode problems. The first column links to the problem in LeetCod
 | [73. Set Matrix Zeroes][lc73]                                                                 | 🟠 Medium  | [![python](res/py.png)][lc73py]                                                                       |
 | [74. Search a 2D Matrix][lc74]                                                                | 🟠 Medium  | [![python](res/py.png)][lc74py]                                                                       |
 | [76. Minimum Window Substring][lc76]                                                          | 🔴 Hard    | [![python](res/py.png)][lc76py]                                                                       |
+| [77. Combinations][lc77]                                                                      | 🟠 Medium  | [![rust](res/rs.png)][lc77rs]                                                                         |
 | [78. Subsets][lc78]                                                                           | 🟠 Medium  | [![python](res/py.png)][lc78py]                                                                       |
 | [79. Word Search][lc79]                                                                       | 🟠 Medium  | [![python](res/py.png)][lc79py]                                                                       |
 | [80. Remove Duplicates from Sorted Array II][lc80]                                            | 🟠 Medium  | [![python](res/py.png)][lc80py] [![rust](res/rs.png)][lc80rs]                                         |
@@ -720,6 +721,8 @@ Solutions to LeetCode problems. The first column links to the problem in LeetCod
 [lc74py]: leetcode/search-a-2d-matrix.py
 [lc76]: https://leetcode.com/problems/minimum-window-substring/
 [lc76py]: leetcode/minimum-window-substring.py
+[lc77]: https://leetcode.com/problems/combinations/
+[lc77rs]: leetcode/combinations.rs
 [lc78]: https://leetcode.com/problems/subsets/
 [lc78py]: leetcode/subsets.py
 [lc79]: https://leetcode.com/problems/word-search/

leetcode/combinations.rs

@@ -0,0 +1,62 @@
+// 77. Combinations
+// 🟠 Medium
+//
+// https://leetcode.com/problems/combinations/
+//
+// Tags: Backtracking
+
+struct Solution;
+impl Solution {
+    /// Use an internal function that iterates over all digits between the last
+    /// digit added plus one and n adding each of them and checking the length
+    /// of the resulting vector, if the vector has length k, add it to the
+    /// result, otherwise call itself to add the next digit.
+    ///
+    /// Time complexity: O(k^n) - We iterate over k digit, for each digit, the
+    /// decision tree branches into n options.
+    /// Space complexity: O(k^n) - Each call to add_digit will add a new digit
+    /// to the result and we saw the number of calls in the time complexity.
+    ///
+    /// Runtime 7 ms Beats 85.74%
+    /// Memory 2.83 MB Beats 72.88%
+    pub fn combine(n: i32, k: i32) -> Vec<Vec<i32>> {
+        let mut res = vec![];
+        fn add_digit(cur: &mut Vec<i32>, res: &mut Vec<Vec<i32>>, l: usize, n: usize, k: usize) {
+            for i in l..=n {
+                cur.push(i as i32);
+                if cur.len() == k {
+                    res.push(cur.clone());
+                } else {
+                    add_digit(cur, res, i + 1, n, k);
+                }
+                cur.pop();
+            }
+        }
+        let mut cur = vec![];
+        add_digit(&mut cur, &mut res, 1, n as usize, k as usize);
+        res
+    }
+}
+
+// Tests.
+fn main() {
+    let tests = [
+        (
+            4,
+            2,
+            vec![
+                vec![1, 2],
+                vec![1, 3],
+                vec![1, 4],
+                vec![2, 3],
+                vec![2, 4],
+                vec![3, 4],
+            ],
+        ),
+        (1, 1, vec![vec![1]]),
+    ];
+    for t in tests {
+        assert_eq!(Solution::combine(t.0, t.1), t.2);
+    }
+    println!("» All tests passed!")
+}