常见算法的复杂度：
<The progmatic programmer> 中对算法的讲义是简洁的。特别是：
O() notation
- O(1) access element in array; Hash
- O(lgn) binary search, traveling a binary tree
- O(n) exhaustive search, finding the max
- O(nlogn) divide and conqure, quick sort/heapsort
- O(n^2) selection and insertion sorts. bubble sort
- O(c^n) traveling salesman problem, set partitioning,permutations