0077. Combinations

Author: coderbean

markdown/0077. Combinations.md

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+### [77\. Combinations](https://leetcode.com/problems/combinations/)
+
+Difficulty: **Medium**
+
+
+Given two integers _n_ and _k_, return all possible combinations of _k_ numbers out of 1 ... _n_.
+
+**Example:**
+
+```
+Input: n = 4, k = 2
+Output:
+[
+  [2,4],
+  [3,4],
+  [2,3],
+  [1,2],
+  [1,3],
+  [1,4],
+]
+```
+
+
+#### Solution
+
+Language: **Java**
+
+```java
+class Solution {
+    public List<List<Integer>> combine(int n, int k) {
+        List<List<Integer>> result = new ArrayList<List<Integer>>();
+        combine(result, new ArrayList<Integer>(), 1, n, k);
+        return result;
+    }
+​
+    public void combine(List<List<Integer>> result, List<Integer> item, int start, int n, int k) {
+        if (k == 0) {
+            result.add(new ArrayList<Integer>(item));
+            return;
+        }
+        for (int i = start; i <= n; i++) {
+            item.add(i);
+            combine(result, item, i + 1, n, k - 1);
+            item.remove(item.size() - 1);
+        }
+    }
+}
+```
+![](https://raw.githubusercontent.com/PicGoBed/PicBed/master/20190724214825.png)

src/main/java/leetcode/_77_/Main.java

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+package leetcode._77_;
+
+/**
+ * Created by zhangbo54 on 2019-03-04.
+ */
+public class Main {
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        System.out.println(solution.combine(4, 2));
+    }
+}
+

src/main/java/leetcode/_77_/Solution.java

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+package leetcode._77_;
+
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+    public List<List<Integer>> combine(int n, int k) {
+        List<List<Integer>> result = new ArrayList<List<Integer>>();
+        combine(result, new ArrayList<Integer>(), 1, n, k);
+        return result;
+    }
+
+    public void combine(List<List<Integer>> result, List<Integer> item, int start, int n, int k) {
+        if (k == 0) {
+            result.add(new ArrayList<Integer>(item));
+            return;
+        }
+        for (int i = start; i <= n; i++) {
+            item.add(i);
+            combine(result, item, i + 1, n, k - 1);
+            item.remove(item.size() - 1);
+        }
+    }
+}

src/main/java/leetcode/_77_/solution.md

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+### [77\. Combinations](https://leetcode.com/problems/combinations/)
+
+Difficulty: **Medium**
+
+
+Given two integers _n_ and _k_, return all possible combinations of _k_ numbers out of 1 ... _n_.
+
+**Example:**
+
+```
+Input: n = 4, k = 2
+Output:
+[
+  [2,4],
+  [3,4],
+  [2,3],
+  [1,2],
+  [1,3],
+  [1,4],
+]
+```
+
+
+#### Solution
+
+Language: **Java**
+
+```java
+class Solution {
+    public List<List<Integer>> combine(int n, int k) {
+        List<List<Integer>> result = new ArrayList<List<Integer>>();
+        combine(result, new ArrayList<Integer>(), 1, n, k);
+        return result;
+    }
+​
+    public void combine(List<List<Integer>> result, List<Integer> item, int start, int n, int k) {
+        if (k == 0) {
+            result.add(new ArrayList<Integer>(item));
+            return;
+        }
+        for (int i = start; i <= n; i++) {
+            item.add(i);
+            combine(result, item, i + 1, n, k - 1);
+            item.remove(item.size() - 1);
+        }
+    }
+}
+```
+![](https://raw.githubusercontent.com/PicGoBed/PicBed/master/20190724214825.png)