Merge pull request #2073 from Lozakaka/patch-9

adding java iteration

problems/0538.把二叉搜索树转换为累加树.md

@@ -177,6 +177,8 @@ public:
 
 
 ## Java 
+**递归**
+
 ```Java
 class Solution {
     int sum;
@@ -198,6 +200,42 @@ class Solution {
     }
 }
 ```
+**迭代**
+
+```Java
+class Solution {
+    //DFS iteraion統一迭代法
+    public TreeNode convertBST(TreeNode root) {
+        int pre = 0;
+        Stack<TreeNode> stack = new Stack<>();
+        if(root == null) //edge case check
+            return null;
+
+        stack.add(root);
+
+        while(!stack.isEmpty()){
+            TreeNode curr = stack.peek();
+            //curr != null的狀況，只負責存node到stack中
+            if(curr != null){ 
+                stack.pop();
+                if(curr.left != null)       //左
+                    stack.add(curr.left);
+                stack.add(curr);            //中
+                stack.add(null);
+                if(curr.right != null)      //右
+                    stack.add(curr.right);
+            }else{
+            //curr == null的狀況，只負責做單層邏輯
+                stack.pop();
+                TreeNode temp = stack.pop();
+                temp.val += pre;
+                pre = temp.val;
+            }
+        }
+        return root;
+    }
+}
+```
 
 ## Python 
 递归法(版本一)