feat(backtracking): add 39_combination_sum.rs

Author: whisperpine

README.md

@@ -145,3 +145,4 @@ so that you can use `just tf TEST` command to test.
 | Problem | Difficulty | Solution |
 | - | - | - |
 | 78. Subsets | Medium | [78_subsets.rs](./tests/78_subsets.rs) |
+| 39. Combination Sum | Medium | [39_combination_sum.rs](./tests/39_combination_sum.rs) |

tests/39_combination_sum.rs

@@ -0,0 +1,44 @@
+// 39. Combination Sum
+// https://leetcode.com/problems/combination-sum/description/
+// Topics: Backtracking.
+// Difficulty: Medium.
+
+#[test]
+fn test_39_combination_sum() {}
+
+#[derive(Debug)]
+pub struct Solution;
+
+// ---------------------------------
+// copy to leetcode starts from here
+// ---------------------------------
+
+impl Solution {
+    pub fn combination_sum(candidates: Vec<i32>, target: i32) -> Vec<Vec<i32>> {
+        let mut result: Vec<Vec<i32>> = vec![];
+        let mut current: Vec<i32> = vec![];
+        find_combinations(0, target, candidates.as_ref(), &mut current, &mut result);
+        result
+    }
+}
+
+fn find_combinations(
+    index: usize,
+    target: i32,
+    candidates: &[i32],
+    current: &mut Vec<i32>,
+    result: &mut Vec<Vec<i32>>,
+) {
+    if target == 0 {
+        result.push(current.to_vec());
+        return;
+    }
+
+    for i in index..candidates.len() {
+        if candidates[i] <= target {
+            current.push(candidates[i]);
+            find_combinations(i, target - candidates[i], candidates, current, result);
+            current.pop();
+        }
+    }
+}