https://oj.leetcode.com/problems/path-sum-ii/
https://oj.leetcode.com/problems/sum-root-to-leaf-numbers/
simpified
modify constructor
https://oj.leetcode.com/problems/length-of-last-word/  strstream not include in LeetCode
string stream was included in LeetCode
use two pointers
https://oj.leetcode.com/problems/merge-two-sorted-lists/
https://oj.leetcode.com/problems/insertion-sort-list/