题解 | #实现二叉树先序，中序和后序遍历#

使用递归解决

/**
 * 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<int> prev;
        vector<int> mid;
        vector<int> post;
        
        preorder(root, prev);
        midorder(root, mid);
        postorder(root, post);
        
        return {prev, mid, post};
        
    }
    
    void preorder(TreeNode* root, vector<int>& prev){
        if(root == nullptr){
            return;
        }
        
        prev.push_back(root->val);
        preorder(root->left, prev);
        preorder(root->right, prev);
    }
    
    void midorder(TreeNode* root, vector<int>& mid){
        if(root == nullptr){
            return;
        }
        
        midorder(root->left, mid);
        mid.push_back(root->val);
        midorder(root->right, mid);
    }
    
    void postorder(TreeNode* root, vector<int>& post){
        if(root == nullptr){
            return;
        }
        
        postorder(root->left, post);
        postorder(root->right, post);
        post.push_back(root->val);
    }
    
};