题解 | #将二叉搜索树改为累加树#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return TreeNode类
     */
    public TreeNode convertBST(TreeNode root) {
        // write code here

        if (null == root || (null == root.left && null == root.right)) {
            return root;
        }
        Stack<TreeNode> stack = new Stack<>();
        ArrayList<Integer> arr = new ArrayList<>();
        TreeNode node = root;
        while (null != node) {
            stack.push(node);
            node = node.left;
        }
        while (!stack.isEmpty()) {
            node = stack.pop();
            arr.add(node.val);
            if (null != node.right) {
                node = node.right;
                while (null != node) {
                    stack.push(node);
                    node = node.left;
                }
            }
        }
        for (int i = arr.size() - 2; i > -1; i--) {
            arr.set(i, arr.get(i) + arr.get(i + 1));
        }
        int index = 0;
        node = root;
        while (null != node) {
            stack.push(node);
            node = node.left;
        }
        while (!stack.isEmpty()) {
            node = stack.pop();
            node.val = arr.get(index);
            index++;
            arr.add(node.val);
            if (null != node.right) {
                node = node.right;
                while (null != node) {
                    stack.push(node);
                    node = node.left;
                }
            }
        }
        return root;
    }
}