938. 二叉搜索树的范围和

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class Solution {
    int ans = 0;
    public int rangeSumBST(TreeNode root, int low, int high) {
        dfs(root, low, high);
        return ans;
    }

    public void dfs(TreeNode root, int low, int high){
        if(root == null){
            return;
        }
        dfs(root.left, low, high);
        if(root.val >=low && root.val <= high){
            ans += root.val;
        }
        dfs(root.right, low, high);
    }
}