题解 | #二叉搜索树与双向链表#

import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {

    public TreeNode Convert(TreeNode pRootOfTree) {
        if (pRootOfTree == null) {
            return pRootOfTree;
        }
        
        TreeNode node = postOrder(pRootOfTree, true);
        while (node.left != null) {
            node = node.left;
        }

        return node;
    }

    private TreeNode postOrder(TreeNode root, boolean isLeft) {
        if (root == null) {
            return root;
        }

        TreeNode nodeLeft = postOrder(root.left, true);
        TreeNode nodeRight = postOrder(root.right, false);
        if (nodeLeft != null) {
            nodeLeft.right = root;
            root.left = nodeLeft;
        }

        if (nodeRight != null) {
            nodeRight.left = root;
            root.right = nodeRight;
        }

        if (isLeft) {
            return nodeRight == null ? root: nodeRight;
        } else {
            return nodeLeft == null ? root : nodeLeft;
        }
    }
}