【重点】538.把二叉树转化为累加树

题目

法1：反中序遍历（使用全局变量）

Python

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def __init__(self):
        self.pre = 0

    def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        self.dfs(root)

        return root

    def dfs(self, root):
        if not root:
            return
        self.dfs(root.right)
        self.pre += root.val
        root.val = self.pre
        self.dfs(root.left)

Java

注意：Java中函数中传递int变量是值传递，不是引用传递！！！所以需要定义全局变量！！！

class Solution {
    public int preSum = 0;
    public TreeNode convertBST(TreeNode root) {
        if (root == null) {
            return root;
        }
        convert(root); // 右-根-左
        return root;
    }

    public void convert(TreeNode root) {
        if (root == null) {
            return;
        }
        convert(root.right);
        preSum += root.val;
        root.val = preSum;
        convert(root.left);
    }
}

法2：反中序遍历（不用全局变量）

Java

class Solution {
    public TreeNode convertBST(TreeNode root) {
        if (root == null) {
            return root;
        }
        convert(root, 0);
        return root;
    }

    public int convert(TreeNode root, int preSum) {
        if (root == null) {
            return preSum;
        }
        root.val = convert(root.right, preSum) + root.val;
        return convert(root.left, root.val);
    }
}