update 二叉树的统一迭代法.md about rust

Author: fwqaaq

problems/二叉树的统一迭代法.md

@@ -666,6 +666,83 @@ object Solution {
   }
 }
 ```
+
+rust:
+
+```rust
+impl Solution{
+    // 前序
+    pub fn preorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
+        let mut res = vec![];
+        let mut stack = vec![];
+        if root.is_some(){
+            stack.push(root);
+        }
+        while !stack.is_empty(){
+            if let Some(node) = stack.pop().unwrap(){
+                if node.borrow().right.is_some(){
+                    stack.push(node.borrow().right.clone());
+                }
+                if node.borrow().left.is_some(){
+                    stack.push(node.borrow().left.clone());
+                }
+                stack.push(Some(node));
+                stack.push(None);
+            }else{
+                res.push(stack.pop().unwrap().unwrap().borrow().val);
+            }
+        }
+        res
+    }
+    // 中序
+        pub fn inorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
+        let mut res = vec![];
+        let mut stack = vec![];
+        if root.is_some() {
+            stack.push(root);
+        }
+        while !stack.is_empty() {
+            if let Some(node) = stack.pop().unwrap() {
+                if node.borrow().right.is_some() {
+                    stack.push(node.borrow().right.clone());
+                }
+                stack.push(Some(node.clone()));
+                stack.push(None);
+                if node.borrow().left.is_some() {
+                    stack.push(node.borrow().left.clone());
+                }
+            } else {
+                res.push(stack.pop().unwrap().unwrap().borrow().val);
+            }
+        }
+        res
+    }
+    // 后序
+    pub fn postorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
+        let mut res = vec![];
+        let mut stack = vec![];
+        if root.is_some() {
+            stack.push(root);
+        }
+        while !stack.is_empty() {
+            if let Some(node) = stack.pop().unwrap() {
+                stack.push(Some(node.clone()));
+                stack.push(None);
+                if node.borrow().right.is_some() {
+                    stack.push(node.borrow().right.clone());
+                }
+                if node.borrow().left.is_some() {
+                    stack.push(node.borrow().left.clone());
+                }
+            } else {
+                res.push(stack.pop().unwrap().unwrap().borrow().val);
+            }
+        }
+        res
+    }
+}
+```
+
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