Merge pull request #1105 from xiaofei-2020/tree24

添加（0098.验证二叉搜索树.md）：增加typescript版本

problems/0098.验证二叉搜索树.md

@@ -526,6 +526,48 @@ var isValidBST = function (root) {
 };
 ```
 
+## TypeScript
+
+> 辅助数组解决：
+
+```typescript
+function isValidBST(root: TreeNode | null): boolean {
+    const traversalArr: number[] = [];
+    function inorderTraverse(root: TreeNode | null): void {
+        if (root === null) return;
+        inorderTraverse(root.left);
+        traversalArr.push(root.val);
+        inorderTraverse(root.right);
+    }
+    inorderTraverse(root);
+    for (let i = 0, length = traversalArr.length; i < length - 1; i++) {
+        if (traversalArr[i] >= traversalArr[i + 1]) return false;
+    }
+    return true;
+};
+```
+
+> 递归中解决：
+
+```typescript
+function isValidBST(root: TreeNode | null): boolean {
+    let maxVal = -Infinity;
+    function inorderTraverse(root: TreeNode | null): boolean {
+        if (root === null) return true;
+        let leftValid: boolean = inorderTraverse(root.left);
+        if (!leftValid) return false;
+        if (maxVal < root.val) {
+            maxVal = root.val
+        } else {
+            return false;
+        }
+        let rightValid: boolean = inorderTraverse(root.right);
+        return leftValid && rightValid;
+    }
+    return inorderTraverse(root);
+};
+```
+
 
 
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