938. Range Sum of BST

O(n) time

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int rangeSumBST(TreeNode* root, int L, int R) {
        if (!root) return 0;
        int res = L <= root->val && root->val <= R ? root->val : 0;
        if (root->val > L) { // 必须是大于L
            res += rangeSumBST(root->left, L, R);
        }
        if (root->val < R) {
            res += rangeSumBST(root->right, L, R);
        }
        return res;
    }
};