1038. Binary Search Tree to Greater Sum Tree

Given the root of a binary search tree with distinct values, modify it so that every node has a new value equal to the sum of the values of the original tree that are greater than or equal to node.val.

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.

Example 1:

Input: [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]

Note:

1. The number of nodes in the tree is between 1 and 100.

2. Each node will have value between 0 and 100.

3. The given tree is a binary search tree.

// Reverse In-Order Traversal
void helper(TreeNode*& node, int& preSum) {
    if (!node) return;
    helper(node->right, preSum);
    node->val += preSum;
    preSum = node->val;
    helper(node->left, preSum);
    return;
}
TreeNode* bstToGst(TreeNode* root) { // time: O(n); space: O(h)
    int preSum = 0;
    helper(root, preSum);
    return root;
}