如何改进O(n^2)的算法

改进到O(n)

1. hash table
2. bit manipulation
3. two pointers(i，j在首，或者i, j 一头一尾)
4. dp 或者greedy

见gas station https://leetcode.com/problems/gas-station/. 不需要 在i ==0不满足条件之后，继续试探i == 1, 试探i == k + 1即可

改进到O(nlogn)

1. binary search
2. sort