LeetCode 94. 二叉树的中序遍历（中序遍历）

示例:

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

输出: [1,3,2]

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ans;
        inorder(root, ans);
        return ans;
    }
    void inorder(TreeNode* root, vector<int> &ans)
    {
        if(root == NULL)
            return;
        inorder(root->left, ans);
        ans.push_back(root->val);
        inorder(root->right, ans);
    }
};

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ans;
        stack<TreeNode*> stk;
        while(root || !stk.empty())
        {
            while(root)
            {
                stk.push(root);
                root = root->left;
            }
            root = stk.top();
            ans.push_back(root->val);
            stk.pop();
            root = root->right;
        }
        return ans;
    }
};