class Solution {
public:
    int trailingZeroes(int n) {
        int res = 0;
        while (n) {
            res += n / 5;
            n /= 5;
        }
        return res;
    }
};
class Solution {
public:
    int trailingZeroes(int n) {
        return n == 0 ? 0 : n / 5 + trailingZeroes(n / 5);
    }
};
return n/5 + n/25 + n/125 + n/625 + n/3125+...;
n/25 可以看作 (n/5)/5

• http://www.cnblogs.com/grandyang/p/4219878.html
• http://www.purplemath.com/modules/factzero.htm
• https://discuss.leetcode.com/topic/6848/my-explanation-of-the-log-n-solution