Given therootnode of a binary search tree, returnthe sum of values of all nodes with a value in the range[low, high].

Example 1:

Input: root = [10,5,15,3,7,null,18], low = 7, high = 15
Output: 32

Example 2:

Input: root = [10,5,15,3,7,13,18,1,null,6], low = 6, high = 10
Output: 23

Constraints:

• The number of nodes in the tree is in the range[1, 2 * 104]
  .
• 1 <= Node.val <= 105
• 1 <= low <= high <= 105
• AllNode.valareunique.

class Solution {
    int res = 0;
    public int rangeSumBST(TreeNode root, int L, int R) {
        dfs(root, L, R);
        return res;
    }
    public void dfs(TreeNode root, int L, int R) {
        if(root == null
 
) return;
        if(root.val >= L && root.val <= R) res += root.val;
        if(L < root.val) dfs(root.left, L, R);
        if(R > root.val) dfs(root.right, L, R);
    }
}

Bst：左小於root，右大於root。判斷每個點是不是在區間，然後判斷往左還是往右。