solve 39.combination-sum

Author: andavid

vscode/39.combination-sum.java

@@ -0,0 +1,74 @@
+/*
+ * @lc app=leetcode id=39 lang=java
+ *
+ * [39] Combination Sum
+ *
+ * https://leetcode.com/problems/combination-sum/description/
+ *
+ * algorithms
+ * Medium (50.27%)
+ * Likes:    2327
+ * Dislikes: 72
+ * Total Accepted:    384.4K
+ * Total Submissions: 763.7K
+ * Testcase Example:  '[2,3,6,7]\n7'
+ *
+ * Given a set of candidate numbers (candidates) (without duplicates) and a
+ * target number (target), find all unique combinations in candidates where the
+ * candidate numbers sums to target.
+ *
+ * The same repeated number may be chosen from candidates unlimited number of
+ * times.
+ *
+ * Note:
+ *
+ *
+ * All numbers (including target) will be positive integers.
+ * The solution set must not contain duplicate combinations.
+ *
+ *
+ * Example 1:
+ *
+ *
+ * Input: candidates = [2,3,6,7], target = 7,
+ * A solution set is:
+ * [
+ * ⁠ [7],
+ * ⁠ [2,2,3]
+ * ]
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: candidates = [2,3,5], target = 8,
+ * A solution set is:
+ * [
+ * [2,2,2,2],
+ * [2,3,3],
+ * [3,5]
+ * ]
+ *
+ *
+ */
+class Solution {
+  public List<List<Integer>> combinationSum(int[] candidates, int target) {
+    List<List<Integer>> list = new ArrayList<>();
+    backtrace(list, new ArrayList<Integer>(), candidates, target, 0);
+    return list;
+  }
+
+  public void backtrace(List<List<Integer>> list, List<Integer> track, int[] candidates, int remain, int start) {
+    if (remain < 0) return;
+    if (remain == 0) {
+      list.add(new ArrayList<Integer>(track));
+    } else {
+      for (int i = start; i < candidates.length; i++) {
+        track.add(candidates[i]);
+        backtrace(list, track, candidates, remain - candidates[i], i);
+        track.remove(track.size() - 1);
+      }
+    }
+  }
+}
+