每周 ARTS 第 33 期

root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8

      10
     /  \
    5   -3
   / \    \
  3   2   11
 / \   \
3  -2   1

返回 3。和等于 8 的路径有:

1.  5 -> 3
2.  5 -> 2 -> 1
3.  -3 -> 11

class Sulution {
    public int pathSum(TreeNode root, int sum) {
        if (root == null) {
            return 0;
        }
        return count(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum);
    }
    private int count(TreeNode root, int sum) {
        if (root == null) {
            return 0;
        }
        int count = 0;
        if (sum == root.val) {
            count += 1;
        }
        int diff = sum - root.val;
        return count + count(root.left, diff) + count(root.right, diff);
    }
}