538. Convert BST to Greater Tree (Easy)

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

Solution 1: Recursion 39ms

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    int sum = 0;
public:
    TreeNode* convertBST(TreeNode* root) {
        if (!root) return NULL;
        convertBST(root->right);
        root->val += sum;
        sum = root->val;
        convertBST(root->left);
        return root;
    }
};

Solution 2: Iteration 42ms

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