1038.BSTtoGST.cs

// 1038. Binary Search Tree to Greater Sum 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 the 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.
// 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]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    int sum = 0;
    public TreeNode BstToGst(TreeNode root) {
        if(root == null)
            return root;
        BstToGst(root.right);
        
        var temp = root.val;
        sum += temp;
        root.val = sum;
        
        BstToGst(root.left);
        return root;
    }
}


public class Solution {
    int sum;
    public TreeNode BstToGst(TreeNode root) {
        sum = DFSSum(root);
        ConvertToGST(root);
        return root;
    }
    private void ConvertToGST(TreeNode root){
        if(root == null)
            return;
        ConvertToGST(root.left);
        int val = root.val;
        root.val = sum;
        sum -= val;
        ConvertToGST(root.right);
    } 
    private int DFSSum(TreeNode root) {
        if(root == null)
            return 0;
        return root.val + DFSSum(root.left) + DFSSum(root.right);
    }
}