Merge pull request #112 from Joshua-Lu/patch-30

更新 0098.验证二叉搜索树 Java版本

problems/0098.验证二叉搜索树.md

@@ -254,21 +254,58 @@ public:
 
 
 Java：
-```java
-class Solution {
-    public boolean isValidBST(TreeNode root) {
-        return isValidBST_2(root, Long.MIN_VALUE, Long.MAX_VALUE);
-    }
 
-    public boolean isValidBST_2 (TreeNode root, long lo, long hi) {
+```Java
-        if (root == null) return true;
+class Solution {
-        if (root.val >= hi || root.val <= lo) return false;
+    // 递归
-        return isValidBST_2(root.left,lo,root.val) && isValidBST_2(root.right,root.val,hi);
+    TreeNode max;
+    public boolean isValidBST(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        // 左
+        boolean left = isValidBST(root.left);
+        if (!left) {
+            return false;
+        }
+        // 中
+        if (max != null && root.val <= max.val) {
+            return false;
+        }
+        max = root;
+        // 右
+        boolean right = isValidBST(root.right);
+        return right;
+    }
+}
+
+class Solution {
+    // 迭代
+    public boolean isValidBST(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        Stack<TreeNode> stack = new Stack<>();
+        TreeNode pre = null;
+        while (root != null || !stack.isEmpty()) {
+            while (root != null) {
+                stack.push(root);
+                root = root.left;// 左
+            }
+            // 中，处理
+            TreeNode pop = stack.pop();
+            if (pre != null && pop.val <= pre.val) {
+                return false;
+            }
+            pre = pop;
+
+            root = pop.right;// 右
+        }
+        return true;
     }
 }
 ```
 
-
 Python：