adding java iteration method

新增java 迭代方法

problems/0669.修剪二叉搜索树.md

@@ -248,6 +248,8 @@ public:
 
 ## Java
 
+**递归**
+
 ```Java
 class Solution {
     public TreeNode trimBST(TreeNode root, int low, int high) {
@@ -269,6 +271,46 @@ class Solution {
 
 ```
 
+**迭代**
+
+```Java
+class Solution {
+    //iteration
+    public TreeNode trimBST(TreeNode root, int low, int high) {
+        if(root == null)
+            return null;
+        while(root != null && (root.val < low || root.val > high)){
+            if(root.val < low)
+                root = root.right;
+            else
+                root = root.left;
+        }
+
+        TreeNode curr = root;
+
+        //deal with root's left sub-tree, and deal with the value smaller than low.
+        while(curr != null){
+            while(curr.left != null && curr.left.val < low){
+                curr.left = curr.left.right;
+            }
+            curr = curr.left;
+        }
+        //go back to root;
+        curr = root;
+
+        //deal with root's righg sub-tree, and deal with the value bigger than high.
+        while(curr != null){
+            while(curr.right != null && curr.right.val > high){
+                curr.right = curr.right.left;
+            }
+            curr = curr.right;
+        }
+        return root;
+    }
+}
+
+````
+
 ## Python
 
 **递归**