94.二叉树的中序遍历

迭代算法：

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