class Solution {
    public int pathSum(TreeNode root, int targetSum) {

        if (root == null) {
            return 0;
        }
        // 以当前节点为根节点
        int now = rootSum(root, targetSum);
        // 考虑左右节点
        int left = pathSum(root.left, targetSum);
        int right = pathSum(root.right, targetSum);
        return now + left + right;
    }

    int rootSum(TreeNode root, int targetSum) {

        if (root == null) {
            return 0;
        }

        int res = 0;
        if (root.val == targetSum) {
            res++;
        }

        int left = rootSum(root.left, targetSum - root.val);
        int right = rootSum(root.right, targetSum - root.val);
        return res + left + right;
    }
}