More problem solved

http://oj.leetcode.com/problems/linked-list-cycle-ii/
http://oj.leetcode.com/problems/best-time-to-buy-and-sell-stock/
http://oj.leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/
http://oj.leetcode.com/problems/best-time-to-buy-and-sell-stock-iii/