★538. Convert binary search tree to cumulative tree

538. Convert binary search tree to cumulative tree

Using class variables in Java is equivalent to using global variables in C++.
It is the first time to use the knowledge of global variables in Java.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    
    int sum = 0;    //设置成员变量sum，每次都累积，这样就可以在遍历左子树的时候累积右边的了。

    public TreeNode convertBST(TreeNode root) {
    
    
        // 先更新右孩子，再更新左孩子
        if(root == null) return root;
        convertBST(root.right);
        sum += root.val;
        root.val = sum;
        convertBST(root.left);
        return root;
    }
}