题解 | #实现二叉树先序，中序和后序遍历#

二叉树递归遍历【前中后序】

（一开始还卡住了。。。。）

/**
 * 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<>>
     */
    void preorder_traversal(std::vector<int> &preorder, TreeNode *root) {
      if (root) {
        preorder.push_back(root->val);
        preorder_traversal(preorder, root->left);
        preorder_traversal(preorder, root->right);
      }
    }
  
    void inorder_traversal(std::vector<int> &inorder, TreeNode *root) {
      if (root) {
        inorder_traversal(inorder, root->left);
        inorder.push_back(root->val);
        inorder_traversal(inorder, root->right);
      }
    }
  
    void postorder_traversal(std::vector<int> &postorder, TreeNode *root) {
      if (root) {
        postorder_traversal(postorder, root->left);
        postorder_traversal(postorder, root->right);
        postorder.push_back(root->val);
      }
    }
  
    vector<vector<int> > threeOrders(TreeNode* root) {
      
      // 前中后序
      std::vector<int> preorder, inorder, postorder;
      
      preorder_traversal(preorder, root);
      inorder_traversal(inorder, root);
      postorder_traversal(postorder, root);
      
      return std::vector<std::vector<int>>{preorder, inorder, postorder};
    }
};