添加 二叉树的统一迭代法.md Java版本boolean标记法

Author: UserXSX

problems/二叉树的统一迭代法.md

@@ -225,7 +225,7 @@ public:
 ## 其他语言版本
 
 ### Java：
-迭代法前序遍历代码如下:
+迭代法前序遍历（空指针标记法）代码如下:
 
 ```java
 class Solution {
@@ -254,7 +254,42 @@ class Solution {
 }
 ```
 
-迭代法中序遍历代码如下:
+迭代法前序遍历（boolean标记法）代码如下:
+
+```java
+class Solution {
+    public List<Integer> preorderTraversal(TreeNode root) {
+        List<Integer> res = new LinkedList<>();
+        if (root == null) {
+            return res;
+        }
+        Stack<Object[]> stack = new Stack<>();
+        stack.push(new Object[]{root, false});
+
+        while (!stack.empty()) {
+            TreeNode node = (TreeNode) stack.peek()[0];
+            Boolean visited = (Boolean) stack.peek()[1];
+            stack.pop();
+
+            if (visited) {
+                res.add(node.val);
+            } else {  // 先序遍历：中左右，入栈顺序相反
+                if (node.right != null) {
+                    stack.push(new Object[]{node.right, false});
+                }
+                if (node.left != null) {
+                    stack.push(new Object[]{node.left, false});
+                }
+                stack.push(new Object[]{node, true});
+            }
+        }
+        return res;
+    }
+}
+```
+
+迭代法中序遍历（空指针标记法）代码如下:
+
 ```java
 class Solution {
 public List<Integer> inorderTraversal(TreeNode root) {
@@ -281,7 +316,41 @@ public List<Integer> inorderTraversal(TreeNode root) {
 }
 ```
 
-迭代法后序遍历代码如下:
+迭代法中序遍历（boolean标记法）代码如下:
+
+```java
+class Solution {
+    public List<Integer> inorderTraversal(TreeNode root) {
+        List<Integer> res = new LinkedList<>();
+        if (root == null) {
+            return res;
+        }
+        Stack<Object[]> stack = new Stack<>();
+        stack.push(new Object[]{root, false});
+        while (!stack.empty()) {
+            TreeNode node = (TreeNode) stack.peek()[0];
+            Boolean visited = (Boolean) stack.peek()[1];
+            stack.pop();
+
+            if (visited) {
+                res.add(node.val);
+            } else {  // 中序遍历：左中右，入栈顺序相反
+                if (node.right != null) {
+                    stack.push(new Object[]{node.right, false});
+                }
+                stack.push(new Object[]{node, true});
+                if (node.left != null) {
+                    stack.push(new Object[]{node.left, false});
+                }
+            }
+        }
+        return res;
+    }
+}
+```
+
+迭代法后序遍历（空指针标记法）代码如下:
+
 ```java
 class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
@@ -309,6 +378,40 @@ class Solution {
 }
 ```
 
+迭代法后序遍历（boolean标记法）代码如下:
+
+```java
+class Solution {
+    public List<Integer> postorderTraversal(TreeNode root) {
+        List<Integer> res = new LinkedList<>();
+        if (root == null) {
+            return res;
+        }
+        Stack<Object[]> stack = new Stack<>();
+        stack.push(new Object[]{root, false});
+
+        while (!stack.empty()) {
+            TreeNode node = (TreeNode) stack.peek()[0];
+            Boolean visited = (Boolean) stack.peek()[1];
+            stack.pop();
+
+            if (visited) {
+                res.add(node.val);
+            } else {  // 后序遍历：左右中，入栈顺序相反
+                stack.push(new Object[]{node, true});
+                if (node.right != null) {
+                    stack.push(new Object[]{node.right, false});
+                }
+                if (node.left != null) {
+                    stack.push(new Object[]{node.left, false});
+                }
+            }
+        }
+        return res;
+    }
+}
+```
+
 ### Python：
 
 > 迭代法前序遍历（空指针标记法）：