update 102

Author: andavid

note/102/README.md

@@ -25,7 +25,7 @@ return its level order traversal as:
 ]
 ```
 
-## 思路
+## 思路一
 
 使用一个队列。先压入根节点和一个空白节点，然后依次弹出节点：
 * 如果节点不为空，把当前值加到列表，然后把节点的左右孩子节点压入队列尾部（如果孩子节点不为空）
@@ -76,5 +76,52 @@ class Solution {
 }
 ```
 
+## 思路二
+
+在思路一的基础上进行优化。思路一通过往队列里添加空白节点作为每一层的分隔。
+其实当前队列的长度就是该层节点的个数，通过 for 循环往队列里压入下一层节点。
+
+## [完整代码][src2]
+
+```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>> list = new ArrayList<>();
+    if (root == null) return list;
+    
+    Queue<TreeNode> queue = new LinkedList<TreeNode>();
+    queue.offer(root);
+
+    while (!queue.isEmpty()) {
+      List<Integer> temp = new ArrayList<>();
+      int size = queue.size();
+      for (int i = 0; i < size; i++) {
+        TreeNode node = queue.peek();
+        if (node.left != null) {
+          queue.offer(node.left);
+        }
+        if (node.right != null) {
+          queue.offer(node.right);
+        }
+        temp.add(queue.poll().val);
+      }
+      list.add(temp);
+    }
+
+    return list;
+  }
+}
+```
+
 [title]: https://leetcode.com/problems/binary-tree-level-order-traversal
 [src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_102/Solution.java
+[src2]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_102/Solution2.java

src/com/andavid/leetcode/_102/Solution2.java

@@ -0,0 +1,65 @@
+import java.util.*;
+
+class Solution2 {
+
+  public List<List<Integer>> levelOrder(TreeNode root) {
+    List<List<Integer>> list = new ArrayList<>();
+    if (root == null) return list;
+
+    Queue<TreeNode> queue = new LinkedList<TreeNode>();
+    queue.offer(root);
+
+    while (!queue.isEmpty()) {
+      List<Integer> temp = new ArrayList<>();
+      int size = queue.size();
+      for (int i = 0; i < size; i++) {
+        TreeNode node = queue.peek();
+        if (node.left != null) {
+          queue.offer(node.left);
+        }
+        if (node.right != null) {
+          queue.offer(node.right);
+        }
+        temp.add(queue.poll().val);
+      }
+      list.add(temp);
+    }
+
+    return list;
+  }
+
+  public static class TreeNode {
+    int val;
+    TreeNode left;
+    TreeNode right;
+    TreeNode(int x) {
+      val = x;
+    }
+  }
+
+  public static void main(String[] args) {
+    TreeNode root = new TreeNode(3);
+    TreeNode node1 = new TreeNode(9);
+    TreeNode node2 = new TreeNode(20);
+    TreeNode node3 = new TreeNode(15);
+    TreeNode node4 = new TreeNode(7);
+    root.left = node1;
+    root.right = node2;
+    node1.left = node1.right = null;
+    node2.left = node3;
+    node2.right = node4;
+    node3.left = node3.right = null;
+    node4.left = node4.right = null;
+
+    Solution solution = new Solution();
+    List<List<Integer>> list = solution.levelOrder(root);
+    for (int i = 0; i < list.size(); i++) {
+      List<Integer> level = list.get(i);
+      for (int val : level) {
+        System.out.print(val + " ");
+      }
+      System.out.println("");
+    }
+  }
+
+}