[Swift]LeetCode538. 把二叉搜索树转换为累加树 | Convert BST to Greater Tree

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Given 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 sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   
           2     13

Output: The root of a Greater Tree like this:
             18
            /   
          20     13

---

给定一个二叉搜索树（Binary Search Tree），把它转换成为累加树（Greater Tree)，使得每个节点的值是原来的节点值加上所有大于它的节点值之和。

例如：

输入: 二叉搜索树:
              5
            /   
           2     13

输出: 转换为累加树:
             18
            /   
          20     13

---

80ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    private var sum = 0
    func convertBST(_ root: TreeNode?) -> TreeNode? {
       // in-order dfs
        if root != nil {
        convertBST(root?.right)
        sum += root!.val
        root!.val = sum
        convertBST(root?.left)
        }
        return root
    }
}

---

84ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    
    func convertBST(_ root: TreeNode?) -> TreeNode? {
        guard let root = root else { return nil }
        
        traverse(root)
        
        return root
    }
    
    var sum = 0
    
    func traverse(_ root: TreeNode?) {
        guard let root = root else { return }
        
        traverse(root.right)
        sum += root.val
        root.val = sum
        traverse(root.left)
    }
}

---

92ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    
    var sum = 0
    
    func convertBST(_ root: TreeNode?) -> TreeNode? {
        if root == nil {
            return root  
        } 
        convertBST(root!.right)
        root!.val = root!.val + sum
        sum = root!.val
        convertBST(root!.left)
        
        return root        
    }  
}

---

100ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
     func convertBST(_ root: TreeNode?) -> TreeNode? {
        var sum = 0
         var stack = Stack()
         
        var node = root
         
         while !stack.isEmpty || node != nil {
             while node != nil {
                 stack.push(node!)
                 node = node?.right
             }
             
             node = stack.pop()
             sum += node!.val
             node!.val = sum
             
             node = node?.left
         }
         
         return root
    }
}

class Stack {
    private var array = [TreeNode]()
    
    var isEmpty: Bool { return array.isEmpty }
    
    func push(_ node: TreeNode) {
        array.append(node)
    }
    
    func pop() -> TreeNode {
        return array.popLast()!
    }
}

---

120ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func convertBST(_ root: TreeNode?) -> TreeNode? {
        _ = convertBST(root, 0)
        return root
    }

    func convertBST(_ root: TreeNode?, _ num: Int) -> Int {
        guard let root = root else {
            return num
        }
        
        var num = convertBST(root.right, num)
        root.val += num
        num = root.val
        num = convertBST(root.left, num)
        return num
    }
}