程序员面试金典——18.4 2的个数

程序员面试金典——18.4 2的个数

Solution1：经典通法，得牢记啊。。。
此题在《剑指offer》中出现过，里面分析的比较到位
https://blog.csdn.net/allenlzcoder/article/details/79619671

class Count2 {
public:
    int countNumberOf2s(int n) {
        // write code here
        int number = 2;//只要定义不同的number，该算法是通用的
        if(n < 2)
            return 0;
        int count = 0;
        int base = 1;
        int round = n;
        while (round > 0) {
            int weight = round%10;
            round /= 10;
            count += round*base;
            if (weight == number)
                count += (n % base) + 1;
            else if (weight > number)
                count += base;
            base *= 10;
        }
        return count;
    }
};