LeetCode:Convert BST to Greater Tree

Posted 2020-09-05

538. Convert BST to Greater Tree

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /              2     13

Output: The root of a Greater Tree like this:
             18
            /             20     13

思路：注意利用BST中序遍历有序的特点，逆序遍历，在加和的时候一定要注意，先遍历右子树，处理当前节点，再遍历左子树。

 1 void addGreaterSum(TreeNode*t, int* sum,bool* isfirst)
 2 {
 3     if (t == NULL)
 4         return;
 5     addGreaterSum(t->right, sum, isfirst);
 6     if (t->right == NULL&&*isfirst)
 7     {
 8         *isfirst = false;
 9         *sum = t->val;
10     }
11     else
12     {
13         int temp = t->val;
14         t->val += *sum;
15         *sum += temp;
16     }
17 
18     addGreaterSum(t->left, sum, isfirst);
19 }
20 TreeNode* convertBST(TreeNode* root) 
21 {
22     int sum = 0;
23     bool isfirst = true;
24     addGreaterSum(root, &sum, &isfirst);
25     return root;
26 }