在N中找到第一个非零数字!在Java中

Question

如何找到N.N的阶乘的第一个非零数可以在1到2147483647的范围内.输出返回一个整数.

EX:

findFirstNoneZeroNumberOfTheFactorial(4) = 4; // because 4! = 24
findFirstNoneZeroNumberOfTheFactorial(5) = 2; // because 5! = 120

这是我的代码:

import java.math.BigInteger;

public class Solution {
    public static int findFirstNoneZeroNumberOfTheFactorial(int n) {
        BigInteger fact = null;
        for (int i = 1; i <= n; i++) {
            fact = fact.multiply(BigInteger.valueOf(i));
        }
        String sFact = String.valueOf(fact);
        for (int i = sFact.length() - 1; i >= 0; i--) {
            if (sFact.charAt(i) != '0') {
                int result = Character.getNumericValue(sFact.charAt(i));
                return result;
            }
        }
        return 0;
    }
    public static void main(String[] args){
        System.out.println(Solution.findFirstNoneZeroNumberOfTheFactorial(4));
    }
}

但它不起作用!!

Answer 1

您的初始化不正确.

BigInteger fact = BigInteger.ONE;

这不是最佳选择.如果您只需要单个最后一个非零数字,则只跟踪最后一个非零数字而不是整个相乘数字.

let's do some math
x     x!      last-nonzero-digit(x * last-nonzero-digit)
1     1             1*1        => 1
2     2             2*1 = 2    => 2
3     6             3*2 = 6    => 6 
4     24            6*4 = 2(4) => 4
5     120           4*5 = (2)0 => 2 
6     720           2*6 = 1(2) => 2
7     5040          2*7 = 1(4) => 4

我为什么要计算整个乘法？

只需保留最后一个非零数字.