给定一个二叉搜索树（Binary Search Tree），把它转换成为累加树（Greater Tree)，使得每个节点的值是原来的节点值加上所有大于它的节点值之和。
例如：
输入: 二叉搜索树:
5
/ \
2 13
输出: 转换为累加树:
18
/ \
20 13
详见：https://leetcode.com/problems/convert-bst-to-greater-tree/description/
C++：
方法一：
/**
* 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 {
public:
TreeNode* convertBST(TreeNode* root)
{
int sum = 0;
helper(root, sum);
return root;
}
void helper(TreeNode*& node, int& sum)
{
if (!node)
{
return;
}
helper(node->right, sum);
node->val += sum;
sum = node->val;
helper(node->left, sum);
}
};
方法二：
/**
* 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 {
public:
TreeNode* convertBST(TreeNode* root) {
if (!root)
{
return nullptr;
}
int sum = 0;
stack<TreeNode*> stk;
TreeNode *p = root;
while (p || !stk.empty())
{
while (p)
{
stk.push(p);
p = p->right;
}
p = stk.top();
stk.pop();
p->val += sum;
sum = p->val;
p = p->left;
}
return root;
}
};
参考：http://www.cnblogs.com/grandyang/p/6591526.html
538 Convert BST to Greater Tree 把二叉搜索树转换为累加树
原文：https://www.cnblogs.com/xidian2014/p/8910068.html