LeetCode 94. 二叉树的中序遍历

题目链接：https://leetcode-cn.com/problems/binary-tree-inorder-traversal/

给定一个二叉树，返回它的中序 遍历。

/**
 * 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> result;
        const TreeNode *p=root;
        stack<const TreeNode *> s;
        while(!s.empty()||p!=nullptr){
            if(p!=nullptr){
                s.push(p);
                p=p->left;
            }else{
                p=s.top();
                s.pop();
                result.push_back(p->val);
                p=p->right;
            }
        }
        return result;
    }
};