653. Two Sum IV - Input is a BST

653. Two Sum IV - Input is a BST
【思路】：

1. 一般采用深度遍历；
2. 和= 顶点+ 右子树 || 左子树+ 右子树 || 左子树+顶点；
3. 使用中序遍历获得顺序，然后通过前后相加比较；
4. Leetcode 653. Two Sum IV - Input is a BST

    bool findTarget(TreeNode* root, int k) {
        vector<int> res;
        dfs_inorder(root,res);
        int sum=0;
        int len = res.size();
        int i=0;
        int j = len-1;
        cout<<"size: "<<len<<endl;
        while(i<len){
            sum = res[i] + res[j];
            if(sum > k)
                j --;
            else if(sum < k)
                i++;
            else
                return true;
        }
        return false;
        
        
    }
    
// DFS
    void dfs_inorder(TreeNode* root, vector<int>& res){
        if(! root )return;
        //cout<<"val: "<<root->val<<endl;
        dfs_inorder(root->left,res);
        res.push_back(root->val);
        dfs_inorder(root->right,res);
    }