538. Convert BST to Greater Tree

Medium

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]

Constraints:

• The number of nodes in the tree is in the range [0, 10^4].

• -10^4 <= Node.val <= 10^4

• All the values in the tree are unique.

• root is guaranteed to be a valid binary search tree.

Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/

解題

Runtime: 9 ms, faster than 100%

Memory Usage: 6.7 MB, less than 79.7%

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func convertBST(root *TreeNode) *TreeNode {
    sum := 0

    var helper func(*TreeNode)
    helper = func(root *TreeNode) {
        if root == nil { return }

        helper(root.Right)
        sum += root.Val
        root.Val = sum
        helper(root.Left)
    }

    helper(root)

    return root
}