Update README.md

abroroo · web-flow · commit da1b6726f882 · 2023-03-01T15:03:08.000+09:00
diff --git a/README.md b/README.md
@@ -4,6 +4,41 @@ List is periodically updated
 
 ## LeetCode
 
+### #101 Symmetric Tree
+
+```
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {boolean}
+ */
+const isSymmetric = function(root) {
+    // tree with null root considered symmetric
+    if (root == null) return true
+    // check if same first two nodes of the root
+    return isSame(root.left, root.right)
+};
+
+const isSame = (leftN, rightN) => {
+    // if both nodes are null then tree symmetric
+    if (leftN == null && rightN == null) return true
+    // if only one of them null, no longer symmetric
+    if (leftN == null || rightN == null) return false
+    // if two values are not equal , not symmetric
+    if (leftN.val !== rightN.val) return false
+    // recursivly check both outer sides of the seconf node and inner sides respectively all over the height of the tree
+    return isSame(leftN.left, rightN.right) && isSame(leftN.right, rightN.left);
+}
+
+```
 ### #26 Remove Duplicate From Sorted Array
 
 ```
