leetcode 94 二叉树的中序遍历

地址：https://leetcode-cn.com/problems/binary-tree-inorder-traversal/
大意：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) {
        stack<TreeNode*> stk;
        vector<int> res;
        TreeNode *kk = root;
        while(stk.size() != 0 || kk != NULL){
            if(kk != NULL){
                stk.push(kk);
                kk = kk->left;
            }else{
                kk = stk.top();
                stk.pop();
                res.push_back(kk->val);
                kk = kk->right;
            }
        }
        return res;
    }
};