前序

 void frontOrder(TreeNode* root)
    {
        if(!root)    return;
        if(!root->left&&!root->right)
        {
            front.push_back(root->val);
            return;
        }
        front.push_back(root->val);
        if(root->left)    frontOrder(root->left);
        if(root->right)    frontOrder(root->right);
        return;
    }

中序

void middleOrder(TreeNode* root)
    {
        if(!root)    return;
        if(!root->left&&!root->right)
        {
            middle.push_back(root->val);
            return;
        }
        if(root->left)    middleOrder(root->left);
        middle.push_back(root->val);
        if(root->right)    middleOrder(root->right);
        return;
    }

后序

void postOrder(TreeNode* root)
    {
        if(!root)    return;
        if(!root->left&&!root->right)
        {
            post.push_back(root->val);
            return;
        }
        if(root->left)    postOrder(root->left);
        if(root->right)    postOrder(root->right);
        post.push_back(root->val);
        return;
    }