Merge pull request #2055 from Lozakaka/patch-5

Update 0098.验证二叉搜索树.md

problems/0098.验证二叉搜索树.md

@@ -259,6 +259,36 @@ public:
 
 ## Java 
 
+```Java
+//使用統一迭代法
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        Stack<TreeNode> stack = new Stack<>();
+        TreeNode pre = null;
+        if(root != null)
+            stack.add(root);        
+        while(!stack.isEmpty()){
+            TreeNode curr = stack.peek();
+            if(curr != null){
+                stack.pop();
+                if(curr.right != null)
+                    stack.add(curr.right);
+                stack.add(curr);
+                stack.add(null);
+                if(curr.left != null)
+                    stack.add(curr.left);
+            }else{
+                stack.pop();
+                TreeNode temp = stack.pop();
+                if(pre != null && pre.val >= temp.val)
+                    return false;
+                pre = temp;
+            }
+        }
+        return true;
+    }
+}
+```
 ```Java
 class Solution {
     // 递归