【LeetCode】Binary Tree Inorder Traversal (2 solutions)

/**
 * Definition for binary tree
 * 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 == NULL)
            return res;
        inorder(root, res);
        return res;
    }
    void inorder(TreeNode* root, vector<int>& res)
    {
        if(root->left)
            inorder(root->left, res);
        res.push_back(root->val);
        if(root->right)
            inorder(root->right, res);
    }
};

/**
 * Definition for binary tree
 * 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 == NULL)
            return res;
        stack<TreeNode*> s;
        map<TreeNode*, bool> m;
        s.push(root);
        m[root] = true;
        while(root->left)
        {
            s.push(root->left);
            m[root->left] = true;
            root = root->left;
        }
        while(!s.empty())
        {
            TreeNode* top = s.top();
            TreeNode* cur = top;
            //left
            bool tag = false;
            while(cur->left && m.find(cur->left)==m.end())
            {
                tag = true;
                s.push(cur->left);
                m[cur->left] = true;
                cur = cur->left;
            }
            if(tag)
                continue;
            //root
            s.pop();
            res.push_back(top->val);
            //right
            if(top->right)
            {
                s.push(top->right);
                m[top->right] = true;
            }
        }
        return res;
    }
};