Update 0404.左叶子之和.md

problems/0404.左叶子之和.md

@@ -575,6 +575,31 @@ object Solution {
 }
 ```
 
+### Rust
+
+**递归**
+
+```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;
+        if let Some(node) = root {
+            if let Some(left) = &node.borrow().left {
+                if left.borrow().right.is_none() && left.borrow().right.is_none() {
+                    res += left.borrow().val;
+                }
+            }
+            res + Self::sum_of_left_leaves(node.borrow().left.clone())
+                + Self::sum_of_left_leaves(node.borrow().right.clone())
+        } else {
+            0
+        }
+    }
+}
+```
+
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