[LeetCode Daily Question] 538. Convert binary search tree to accumulative tree

[LeetCode Daily Question] 538. Convert binary search tree to accumulative tree

538. Convert binary search tree to accumulative tree

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algorithm idea: tree, right middle left (reverse middle order traversal)

Binary search tree middle order traversal (left middle right) to get the sequence from small to large;
reverse middle order traversal (right, middle left) to get the sequence from large to small;
just use sum for superposition combination;

java code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
    
    
		dfsBST(root);
		return root;
    }
    
	public void dfsBST(TreeNode root) {
    
    
		if(root == null) {
    
    
			return;
		}
		dfsBST(root.right);//反中序遍历,先遍历右子树
		root.val = root.val + sum;//叠加
        sum = root.val;//将值保存,下一个结点计算
		dfsBST(root.left);//反中序遍历,遍历左子树
	}
}