05 August 2008
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:
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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
Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/
先递归到右子树，这样加的值会少一些；然后做相加操作，然后递归到左子树。或者维护一个变量，记录右子树中应该加的sum，然后从右边开始遍历。
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
int sum = 0;
public TreeNode convertBST(TreeNode root) {
inorder(root);
return root;
}
private void inorder(TreeNode node) {
if (node == null) {
return;
}
inorder(node.right); // 直到最右子树
node.val += sum;
sum = node.val; //已经加过了
inorder(node.left); // 最后遍历左子树
}
}
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class Solution {
public TreeNode convertBST(TreeNode root) {
dfs(root, 0);
return root;
}
public int dfs(TreeNode root, int val) {
if(root == null) {
return val;
}
int right = dfs(root.right, val);
int left = dfs(root.left, root.val + right);
root.val = root.val + right;
return left;
}
}