Binary Search Tree to Greater Sum Tree

LeetCode Q 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:

• The number of nodes in the tree is between 1 and 100.
• Each node will have value between 0 and 100.
• The given tree is a binary search tree.

Solution:

DFS utilizing property of BST.

Code:

int sum = 0;
public TreeNode bstToGst(TreeNode root) {
  dfs(root);
  return root;
}

private void dfs (TreeNode root) {
  if (root == null) return;

  dfs(root.right);

  sum += root.val;
  root.val = sum;

  dfs(root.left);
}