39. Combination Sum

Author: coderbean

src/leetcode/_39_/Main.java

@@ -0,0 +1,14 @@
+package leetcode._39_;
+
+
+/**
+ * Created by zhangbo54 on 2019-03-04.
+ */
+public class Main {
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        System.out.println(solution.combinationSum(new int[]{48, 22, 49, 24, 26, 47, 33, 40, 37,
+                39, 31, 46, 36, 43, 45, 34, 28, 20, 29, 25, 41, 32, 23}, 69));
+    }
+}
+

src/leetcode/_39_/Solution.java

@@ -0,0 +1,39 @@
+package leetcode._39_;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
+
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        Set<Integer> candidateSet = Arrays.stream(candidates).boxed().collect(Collectors.toCollection(TreeSet::new));
+        return this.combinationSum(candidateSet, target);
+    }
+
+    // here def a new func to avoid calculate a new Set every time
+    private List<List<Integer>> combinationSum(Set<Integer> candidateSet, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        if (candidateSet.contains(target)) {
+            result.add(new ArrayList<>(Collections.singleton(target)));
+        }
+        for (Integer candidate : candidateSet) {
+            int newTarget = target - candidate;
+            if (newTarget < candidate) {
+                return result;
+            } else {
+                for (List<Integer> list : this.combinationSum(candidateSet, newTarget)) {
+                    // this ensure all result are desc sorted to avoid repetition
+                    if (candidate <= list.get(list.size() - 1)) {
+                        list.add(candidate);
+                        result.add(list);
+                    }
+                }
+            }
+        }
+        return result;
+    }
+}

src/leetcode/_39_/solution.md

@@ -0,0 +1,74 @@
+### [39\. Combination SumCopy for Markdown](https://leetcode.com/problems/combination-sum/)
+
+Difficulty: **Medium**
+
+
+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]
+]
+```
+
+
+#### Solution
+
+Language: **Java**
+
+```java
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        Set<Integer> candidateSet = Arrays.stream(candidates).boxed().collect(Collectors.toCollection(TreeSet::new));
+        return this.combinationSum(candidateSet, target);
+    }
+​
+    // here def a new func to avoid calculate a new Set every time
+    private List<List<Integer>> combinationSum(Set<Integer> candidateSet, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        if (candidateSet.contains(target)) {
+            result.add(new ArrayList<>(Collections.singleton(target)));
+        }
+        for (Integer candidate : candidateSet) {
+            int newTarget = target - candidate;
+            if (newTarget < candidate) {
+                return result;
+            } else {
+                for (List<Integer> list : this.combinationSum(candidateSet, newTarget)) {
+                    // this ensure all result are desc sorted to avoid repetition
+                    if (candidate <= list.get(list.size() - 1)) {
+                        list.add(candidate);
+                        result.add(list);
+                    }
+                }
+            }
+        }
+        return result;
+    }
+}
+```
+![](https://ws1.sinaimg.cn/large/006tKfTcgy1g1bmizhcvoj310j0u0dkq.jpg)