leetcode 94. 二叉树的中序遍历(Binary Tree Inorder Traversal)

目录

• 题目描述：
• 示例:
• 解法：

题目描述：

给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,3,2]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

---

解法：

# define PR pair<TreeNode*, int>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(root){
            stack<PR> stk;
            stk.push({root, 0});
            // state 0 indicate the left subtree not processed
            // state 1 indicate the left subtree has processed
            // state 2 indicate the right subtree has processed
            while(!stk.empty()){
                PR pr = stk.top();
                stk.pop();
                TreeNode* node = pr.first;
                int state = pr.second;
                if(state == 0){
                    if(node->left != NULL){
                        stk.push(pr);
                        stk.push({node->left, 0});
                    }else{
                        stk.push({node, 1});
                    }
                }else if(state == 1){
                    res.push_back(node->val);
                    if(node->right != NULL){
                        stk.push(pr);
                        stk.push({node->right, 0});
                    }else{
                        stk.push({node, 2});
                    }
                }else if(state == 2){
                    while(!stk.empty()){
                        PR pr = stk.top();
                        pr.second++;
                        stk.pop();
                        if(pr.second < 2){
                            stk.push(pr);
                            break;
                        }
                    }
                }
            }
        }
        return res;
    }
};