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;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > threeOrders(TreeNode* root) {
        // write code here
        vector<vector<int> > res(3);
        preOrder(root,res[0]);
        inOrder(root,res[1]);
        posOrder(root,res[2]);
        return res;
    }
    vector<int> preOrder(TreeNode* root,vector<int>& res){
        if(root == nullptr) return {};
        res.push_back(root->val);
        preOrder(root->left, res);
        preOrder(root->right, res);
        return res;
    }
    vector<int> inOrder(TreeNode* root,vector<int>& res){
        if(root == nullptr) return {};
        inOrder(root->left, res);
        res.push_back(root->val);
        inOrder(root->right, res);
        return res;
    }
    vector<int> posOrder(TreeNode* root,vector<int>& res){
        if(root == nullptr) return {};
        posOrder(root->left, res);
        posOrder(root->right, res);
        res.push_back(root->val);
        return res;
    }
};