http://oj.leetcode.com/problems/populating-next-right-pointers-in-each-node/
http://oj.leetcode.com/problems/populating-next-right-pointers-in-each-node-ii/
http://oj.leetcode.com/problems/pascals-triangle/
http://oj.leetcode.com/problems/pascals-triangle-ii/
http://oj.leetcode.com/problems/triangle/