[LeetCode] Flatten Binary Tree to Linked List .java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        flattenAndGetLast(root);
    }
    
    public TreeNode flattenAndGetLast(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode leftLast = flattenAndGetLast(root.left);
        TreeNode rightLast = flattenAndGetLast(root.right);
        if (leftLast != null) {
            TreeNode oldRight = root.right;
            root.right = root.left;
            root.left = null;
            leftLast.right = oldRight;
        }
        return rightLast == null ? (leftLast == null ? root : leftLast) : rightLast;
    }
}