Add solution 102

Author: yanglbme

README.md

@@ -58,6 +58,7 @@ Complete solutions to Leetcode problems, updated daily.
 | 086 | [Partition List](https://github.com/doocs/leetcode/tree/master/solution/086.Partition%20List) | `Linked List`, `Two Pointers` |
 | 092 | [Reverse Linked List II](https://github.com/doocs/leetcode/tree/master/solution/092.Reverse%20Linked%20List%20II) | `Linked List` |
 | 094 | [Binary Tree Inorder Traversal](https://github.com/doocs/leetcode/tree/master/solution/094.Binary%20Tree%20Inorder%20Traversal) | `Hash Table`, `Stack`, `Tree` |
+| 102 | [Binary Tree Level Order Traversal](https://github.com/doocs/leetcode/tree/master/solution/102.Binary%20Tree%20Level%20Order%20Traversal) | `Tree`, `Breadth-first Search` |
 | 127 | [Word Ladder](https://github.com/doocs/leetcode/tree/master/solution/127.Word%20Ladder) | `Breadth-first Search` |
 | 130 | [Surrounded Regions](https://github.com/doocs/leetcode/tree/master/solution/130.Surrounded%20Regions) | `Depth-first Search`, `Breadth-first Search`, `Union Find` |
 | 144 | [Binary Tree Preorder Traversal](https://github.com/doocs/leetcode/tree/master/solution/144.Binary%20Tree%20Preorder%20Traversal) | `Stack`, `Tree` |

solution/102.Binary Tree Level Order Traversal/README.md

@@ -0,0 +1,68 @@
+## 二叉树的层次遍历
+### 题目描述
+
+给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。
+
+例如:
+给定二叉树: `[3,9,20,null,null,15,7]`,
+```
+
+    3
+   / \
+  9  20
+    /  \
+   15   7
+```
+
+返回其层次遍历结果：
+```
+[
+  [3],
+  [9,20],
+  [15,7]
+]
+```
+
+### 解法
+利用队列，存储二叉树结点。`size` 表示每一层的结点数量，也就是内层循环的次数。
+
+```java
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> res = new ArrayList<>();
+        if (root == null) {
+            return res;
+        }
+        
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            int size = queue.size();
+            List<Integer> list = new ArrayList<>();
+            while ((size--) > 0) {
+                TreeNode node = queue.poll();
+                list.add(node.val);
+                if (node.left != null) {
+                    queue.offer(node.left);
+                }
+                if (node.right != null) {
+                    queue.offer(node.right);
+                }
+            }
+            res.add(list);
+        }
+        
+        return res;
+    }
+}
+
+```

solution/102.Binary Tree Level Order Traversal/Solution.java

@@ -0,0 +1,28 @@
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> res = new ArrayList<>();
+        if (root == null) {
+            return res;
+        }
+        
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            int size = queue.size();
+            List<Integer> list = new ArrayList<>();
+            while ((size--) > 0) {
+                TreeNode node = queue.poll();
+                list.add(node.val);
+                if (node.left != null) {
+                    queue.offer(node.left);
+                }
+                if (node.right != null) {
+                    queue.offer(node.right);
+                }
+            }
+            res.add(list);
+        }
+        
+        return res;
+    }
+}