from root to leaf tree traverse problem (直上直下 )

Class2: 从root 往下，（直上直下） path。 key points : to carry 直上直下 path prefix while traversing the tree

小结： 如果必须是 root to leaf， 则base case is root.left == null && root.right == null. corner case is root == null

1. Maximum path sum
2. Path sum to target number