1038.binary-search-tree-to-greater-sum-tree.java

/*
 * @lc app=leetcode id=1038 lang=java
 *
 * [1038] Binary Search Tree to Greater Sum Tree
 *
 * https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/description/
 *
 * algorithms
 * Medium (82.76%)
 * Likes:    229
 * Dislikes: 33
 * Total Accepted:    14.7K
 * Total Submissions: 18.6K
 * Testcase Example:  '[4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]'
 *
 * Given the root of a binary search tree with distinct values, modify it so
 * that every node has a new value equal to the sum of the values of the
 * original tree that are greater than or equal to node.val.
 *
 * As a reminder, a binary search tree is a tree that satisfies these
 * constraints:
 *
 *
 * The left subtree of a node contains only nodes with keys less than the
 * node's key.
 * The right subtree of a node contains only nodes with keys greater than the
 * node's key.
 * Both the left and right subtrees must also be binary search trees.
 *
 *
 *
 *
 * Example 1:
 *
 *
 *
 *
 * Input: [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
 * Output: [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]
 *
 *
 *
 *
 *
 *
 * Note:
 *
 *
 * The number of nodes in the tree is between 1 and 100.
 * Each node will have value between 0 and 100.
 * The given tree is a binary search tree.
 *
 *
 *
 *
 *
 *
 *
 *
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public TreeNode bstToGst(TreeNode root) {
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = root;
    int sum = 0;

    while (cur != null || !stack.empty()) {
      while (cur != null) {
        stack.push(cur);
        cur = cur.right;
      }
      cur = stack.pop();
      sum = cur.val = sum + cur.val;
      cur = cur.left;
    }

    return root;
  }
}