538. Convert BST to Greater Tree

538. Convert BST to Greater Tree

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

题意：

把一个二叉搜索树的每个节点都加上所有节点里面比他大的值，构造成一个更大的二叉搜索树

算法思路：

这里一定要注意二叉搜索树的特点，每个节点的右子树都比节点大，左子树都比节点小

所以每个节点只需要加上他右子树即可

所以这里要先遍历右子树、节点、左子树

把右子树中的内容都放在sum中，依次累加

代码：

package easy;public class ConvertBSTtoGreaterTree {int sum = 0;public TreeNode convertBST(TreeNode root) {convert(root);return root;            }public void convert(TreeNode cur){if(null == cur) return;convert(cur.right);cur.val += sum;sum = cur.val;convert(cur.left);}public class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int x) { val = x; }  }}