114. Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    //post order
    TreeNode pre = null;
    public void flatten(TreeNode root) {
        if(root == null)
            return;
        flatten(root.right);
        flatten(root.left);
        root.right = pre;
        root.left = null;
        pre = root;
    }
}