Convert BST to Greater Tree

题目描述：

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, 104].
-104 <= Node.val <= 104
All the values in the tree are unique.
root is guaranteed to be a valid binary search tree.

思路:

dfs 搜索，右中左序列就行

代码:

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func convertBST(root *TreeNode) *TreeNode {
    var dfs func(root *TreeNode)
    var maxNum int 
    dfs = func(root *TreeNode){
        if root == nil{
            return 
        }
        dfs(root.Right)
        root.Val += maxNum
        maxNum = root.Val
        dfs(root.Left)
        
    }
dfs(root)
return root
}
func max(a,b int)int{
    if a>b{
        return a
    }else{
        return b
    }
}

代码效率：

执行用时：0 ms, 在所有 Go 提交中击败了100.00%的用户
内存消耗：2.2 MB, 在所有 Go 提交中击败了100.00%的用户