Update 0404.左叶子之和.md

problems/0404.左叶子之和.md

@@ -600,6 +600,33 @@ impl Solution {
 }
 ```
 
+**迭代：**
+
+```rust
+use std::cell::RefCell;
+use std::rc::Rc;
+impl Solution {
+    pub fn sum_of_left_leaves(root: Option<Rc<RefCell<TreeNode>>>) -> i32 {
+        let mut res = 0;
+        let mut stack = vec![root];
+        while !stack.is_empty() {
+            if let Some(node) = stack.pop().unwrap() {
+                if let Some(left) = &node.borrow().left {
+                    if left.borrow().left.is_none() && left.borrow().right.is_none() {
+                        res += left.borrow().val;
+                    }
+                    stack.push(Some(left.to_owned()));
+                }
+                if let Some(right) = &node.borrow().right {
+                    stack.push(Some(right.to_owned()));
+                }
+            }
+        }
+        res
+    }
+}
+```
+
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