Add solution 145

Author: yanglbme

README.md

@@ -60,6 +60,7 @@ Complete solutions to Leetcode problems, updated daily.
 | # | Title | Tags |
 |---|---|---|
 | 023 | [Merge k Sorted Lists](https://github.com/yanglbme/leetcode/tree/master/solution/023.Merge%20k%20Sorted%20Lists) | `Linked List`, `Divide and Conquer`, `Heap` |
+| 145 | [Binary Tree Postorder Traversal](https://github.com/yanglbme/leetcode/tree/master/solution/145.Binary%20Tree%20Postorder%20Traversal) | `Stack`, `Tree` |
 
 ## Contributions
 I'm looking for long-term contributors/partners to this repo! Send me PRs if you're interested! See the following:

solution/145.Binary Tree Postorder Traversal/README.md

@@ -0,0 +1,59 @@
+## 二叉树的后序遍历
+### 题目描述
+
+给定一个二叉树，返回它的 后序 遍历。
+
+示例:
+```
+输入: [1,null,2,3]  
+   1
+    \
+     2
+    /
+   3 
+
+输出: [3,2,1]
+```
+
+进阶: 递归算法很简单，你可以通过迭代算法完成吗？
+
+### 解法
+
+- 递归算法
+
+先递归左子树，再递归右子树，最后访问根结点。
+
+```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<Integer> postorderTraversal(TreeNode root) {
+        List<Integer> list = new ArrayList<>();
+        postorderTraversal(root, list);
+        return list;
+    }
+    
+    private void postorderTraversal(TreeNode root, List<Integer> list) {
+        if (root == null) {
+            return;
+        }
+        postorderTraversal(root.left, list);
+        postorderTraversal(root.right, list);
+        list.add(root.val);
+    }
+}
+```
+
+- 非递归算法
+
+
+```java
+
+```

solution/145.Binary Tree Postorder Traversal/Solution.java

@@ -0,0 +1,44 @@
+// 递归版本
+/*
+class Solution {
+    public List<Integer> preorderTraversal(TreeNode root) {
+        List<Integer> list = new ArrayList<>();
+        preorderTraversal(root, list);
+        return list;
+    }
+    
+    private void preorderTraversal(TreeNode root, List<Integer> list) {
+        if (root == null) {
+            return;
+        }
+        list.add(root.val);
+        preorderTraversal(root.left, list);
+        preorderTraversal(root.right, list);
+    }
+}
+
+*/
+
+// 非递归版本
+class Solution {
+    public List<Integer> preorderTraversal(TreeNode root) {
+        List<Integer> list = new ArrayList<>();
+        Stack<TreeNode> stack = new Stack<>();
+        while (root != null) {
+            list.add(root.val);
+            if (root.right != null) {
+                stack.push(root.right);
+            }
+            if (root.left != null) {
+                stack.push(root.left);
+            }
+            if (!stack.isEmpty()) {
+                root = stack.pop();
+            } else {
+                break;
+            }
+        }
+        
+        return list;
+    }
+}