【LeetCode】94. Binary Tree Inorder Traversal

1、题目描述

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
1
 \
  2
  /
3

Output: [1,3,2]
Follow up: Recursive solution is trivial, could you do it iteratively?

2、问题描述：

非递归实现，中序。

3、问题关键：

4、C++代码：

递归

class Solution {
public:
    vector res;
    vector inorderTraversal(TreeNode* root) {
        if (!root) return vector();
        dfs(root);
        return res;
    }
    void dfs(TreeNode* root) {
        if (!root) return;
        dfs(root->left);
        res.push_back(root->val);
        dfs(root->right);
    }
};

模拟法。（stack）

/**
 * 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 inorderTraversal(TreeNode* root) {
        stack st;
        vector res;
        auto now = root;
        while (now || !st.empty()) {
            while (now) {
                st.push(now);
                now = now->left;
            }
            if (!st.empty()) {
                now = st.top();
                st.pop();
                res.push_back(now->val);
                now = now->right;
            }
        }
        return res;
    }
};