递归实现二叉树前、中、后序遍历

注意：
三者的区别就是处理根节点的时间

代码

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型vector<vector<>>
     */
    // 前序遍历
    vector<int> pre;
    vector<int> mid;
    vector<int> after;

    vector<vector<int>> res;

    // 前序遍历
    void preOrders(const TreeNode* root){
        if(root==nullptr){
            return ;
        }
        pre.push_back(root->val);
        preOrders(root->left);
        preOrders(root->right);
    }
    // 中序遍历
    void midOrders(TreeNode* root){
        if(root==nullptr){
            return ;
        }
        midOrders(root->left);
        mid.push_back(root->val);
        midOrders(root->right);
    }

    // 后序遍历
    void afterOrders(TreeNode* root){
        if(root==nullptr){
            return ;
        }
        afterOrders(root->left);
        afterOrders(root->right);
        after.push_back(root->val);

    }


    vector<vector<int> > threeOrders(TreeNode* root) {

        // write code here
        preOrders(root);
        midOrders(root);
        afterOrders(root);

        res.push_back(pre);
        res.push_back(mid);
        res.push_back(after);

        return res;

    }
};