# 538. Convert BST to Greater Tree

Given the `root` of 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.

As a reminder, a  binary search tree  is a tree that satisfies these constraints:

• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/

Example 1:

``````Input: root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
Output: [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]
``````

Example 2:

``````Input: root = [0,null,1]
Output: [1,null,1]
``````

Example 3:

``````Input: root = [1,0,2]
Output: [3,3,2]
``````

Example 4:

``````Input: root = [3,2,4,1]
Output: [7,9,4,10]
``````

Constraints:

• The number of nodes in the tree is in the range `[0, 104]`.
• `-104 <= Node.val <= 104`
• All the values in the tree are unique.
• `root` is guaranteed to be a valid binary search tree.

``````class Solution {
public:
TreeNode* convertBST(TreeNode* root) {
int sum = 0;
helper(root, sum);
return root;
}
void helper(TreeNode*& node, int& sum) {
if (!node) return;
helper(node->right, sum);
node->val += sum;
sum = node->val;
helper(node->left, sum);
}
};
``````

``````class Solution {
public:
TreeNode* convertBST(TreeNode* root) {
if (!root) return NULL;
convertBST(root->right);
root->val += sum;
sum = root->val;
convertBST(root->left);
return root;
}

private:
int sum = 0;
};
``````

``````class Solution {
public:
TreeNode* convertBST(TreeNode* root) {
if (!root) return NULL;
int sum = 0;
stack<TreeNode*> st;
TreeNode *p = root;
while (p || !st.empty()) {
while (p) {
st.push(p);
p = p->right;
}
p = st.top(); st.pop();
p->val += sum;
sum = p->val;
p = p->left;
}
return root;
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/538

https://leetcode.com/problems/convert-bst-to-greater-tree/

https://leetcode.com/problems/convert-bst-to-greater-tree/discuss/100506/Java-Recursive-O(n)-time

https://leetcode.com/problems/convert-bst-to-greater-tree/discuss/100610/c%2B%2B-solution-beats-100

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