Create Combination Sum.java

Combination Sum.java

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+/*
+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]
+]
+
+
+time = O(2^n * n)
+space = O(n)
+*/
+
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        if (candidates == null) {
+            return result;
+        }
+        Arrays.sort(candidates);
+        dfs(result, new ArrayList<>(), 0, target, candidates);
+        return result;
+    }
+
+    private void dfs(List<List<Integer>> result, List<Integer> list, int start, int remain, int[] candidates) {
+        if (remain < 0) {
+            return;
+        }
+        if (remain == 0) {
+            result.add(new ArrayList<>(list));
+            return;
+        }
+
+        for (int i = start; i < candidates.length; i++) {
+            list.add(candidates[i]);
+            dfs(result, list, i, remain - candidates[i], candidates);
+            list.remove(list.size() - 1);
+        }
+    }
+}