leetcode-1038-Binary-Search-Tree-to-Greater-Sum-Tree

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:

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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.

解题思路：递归先搜索完右边节点在计算左边节点。

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    int pre = 0;
public:
    TreeNode* bstToGst(TreeNode* root) {
        if (root != NULL)
        {
            bstToGst(root->right);
            pre = root->val = root->val + pre;
            bstToGst(root->left);
        }
        return root;
    }
};