LeetCode_94_二叉树的中序遍历

题目描述：

给定一个二叉树，返回它的中序 遍历。

示例:

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

输出: [1,3,2]

/**
 * 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> res;
    void inorder(TreeNode* root){
        if(root==NULL)
            return ;
        inorder(root->left);
        res.push_back(root->val);
        inorder(root->right);     
    }
    vector<int> inorderTraversal(TreeNode* root) {
        inorder(root);
        return res;
    }
};

非递归算法：

/**
 * 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> res;
    stack<TreeNode*> temp;
    vector<int> inorderTraversal(TreeNode* root) {
        TreeNode* p=root;
        while(p||!temp.empty()){
            if(p){
                temp.push(p);
                p=p->left;
                continue;
            }else{
                TreeNode* node=temp.top();
                int x=node->val;
                temp.pop();
                res.push_back(x);
                p=node->right;
            }
        }
        return res;
    }
};