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538. Convert BST to Greater Tree

Posted on Edited on In Disqus:
Symbols count in article: 901 Reading time ≈ 1 mins.

right-middle-left reverse order traversal O(n)

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

    int sum = 0;
};
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/**
 * 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 {
public:
    TreeNode* convertBST(TreeNode* root) {
        int sum = 0;
        helper(root, sum);
        return root;
    }

    void helper(TreeNode *root, int &sum) {
        if (!root) return;
        helper(root->right, sum);
        root->val += sum;
        sum = root->val;
        helper(root->left, sum);
    }
};