题解 | #实现二叉树先序，中序和后序遍历#

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
#include <vector>
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型vector<vector<>>
     */
    vector<vector<int>> res;
    vector<int> preorder, inorder, postorder;
    
    void preOrder(TreeNode* root){
        if(!root) return;
        preorder.push_back(root->val);
        preOrder(root->left);
        preOrder(root->right);
    }
    void inOrder(TreeNode* root){
        if(!root) return;
        inOrder(root->left);
        inorder.push_back(root->val);
        inOrder(root->right);
    }
    void postOrder(TreeNode* root){
        if(!root) return;
        postOrder(root->left);
        postOrder(root->right);
        postorder.push_back(root->val);
    }

    vector<vector<int> > threeOrders(TreeNode* root) {
        preOrder(root);
        inOrder(root);
        postOrder(root);
        res.push_back(preorder);
        res.push_back(inorder);
        res.push_back(postorder);
        return res;
    }
};