diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index 7ed9888..4af1c24 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -578,3 +578,5 @@ mod p2140_solving_questions_with_brainpower;
 mod p2873_maximum_value_of_an_ordered_triplet_i;
 
 mod p2874_maximum_value_of_an_ordered_triplet_ii;
+
+mod p1123_lowest_common_ancestor_of_deepest_leaves;
diff --git a/src/problem/p1123_lowest_common_ancestor_of_deepest_leaves.rs b/src/problem/p1123_lowest_common_ancestor_of_deepest_leaves.rs
new file mode 100644
index 0000000..17d2921
--- /dev/null
+++ b/src/problem/p1123_lowest_common_ancestor_of_deepest_leaves.rs
@@ -0,0 +1,98 @@
+/**
+ * [1123] Lowest Common Ancestor of Deepest Leaves
+ */
+pub struct Solution {}
+
+use crate::util::tree::{to_tree, TreeNode};
+
+// submission codes start here
+
+// Definition for a binary tree node.
+// #[derive(Debug, PartialEq, Eq)]
+// pub struct TreeNode {
+//   pub val: i32,
+//   pub left: Option<Rc<RefCell<TreeNode>>>,
+//   pub right: Option<Rc<RefCell<TreeNode>>>,
+// }
+//
+// impl TreeNode {
+//   #[inline]
+//   pub fn new(val: i32) -> Self {
+//     TreeNode {
+//       val,
+//       left: None,
+//       right: None
+//     }
+//   }
+// }
+use std::cell::RefCell;
+use std::cmp::Ordering;
+use std::rc::Rc;
+
+impl Solution {
+    pub fn lca_deepest_leaves(
+        root: Option<Rc<RefCell<TreeNode>>>,
+    ) -> Option<Rc<RefCell<TreeNode>>> {
+        if let Some(root) = root {
+            Self::search(&root).1
+        } else {
+            None
+        }
+    }
+
+    // (子树的深度，lca）
+    fn search(node: &Rc<RefCell<TreeNode>>) -> (i32, Option<Rc<RefCell<TreeNode>>>) {
+        let (left_deep, left_lca) = if let Some(left) = node.borrow().left.as_ref() {
+            Self::search(left)
+        } else {
+            (0, None)
+        };
+
+        let (right_deep, right_lca) = if let Some(right) = node.borrow().right.as_ref() {
+            Self::search(right)
+        } else {
+            (0, None)
+        };
+
+        match left_deep.cmp(&right_deep) {
+            Ordering::Greater => (left_deep + 1, left_lca),
+            Ordering::Less => (right_deep + 1, right_lca),
+            Ordering::Equal => (left_deep + 1, Some(node.clone())),
+        }
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_1123() {
+        let node = Solution::lca_deepest_leaves(to_tree(vec![
+            Some(3),
+            Some(5),
+            Some(1),
+            Some(6),
+            Some(2),
+            Some(0),
+            Some(8),
+            None,
+            None,
+            Some(7),
+            Some(4),
+        ]));
+
+        assert_eq!(to_tree(vec![Some(2), Some(7), Some(4)]), node);
+
+        assert_eq!(
+            to_tree(vec![Some(1)]),
+            Solution::lca_deepest_leaves(to_tree(vec![Some(1)]))
+        );
+        assert_eq!(
+            to_tree(vec![Some(2)]),
+            Solution::lca_deepest_leaves(to_tree(vec![Some(0), Some(1), Some(3), None, Some(2)]))
+        );
+    }
+}