Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.
Example:
Input: The root of a Binary Search Tree like this: 5 / \ 2 13 Output: The root of a Greater Tree like this: 18 / \ 20 13
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思路：采用类似中序遍历的非递归方法，用栈不过是按照“右中左的顺序”来遍历，则当前的值为其前驱的值加上当前节点的值。
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public TreeNode convertBST(TreeNode root) { convertHelper(root); return root; } public void convertHelper(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); int pre = 0; while (root != null || !stack.isEmpty()) { while (root != null) { stack.push(root); root = root.right; } root = stack.pop(); root.val = root.val + pre; pre = root.val; root = root.left; } } }