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中等
数学
字符串
模拟

English Version

题目描述

给定两个以字符串形式表示的非负整数 num1 和 num2,返回 num1 和 num2 的乘积,它们的乘积也表示为字符串形式。

注意:不能使用任何内置的 BigInteger 库或直接将输入转换为整数。

 

示例 1:

输入: num1 = "2", num2 = "3"
输出: "6"

示例 2:

输入: num1 = "123", num2 = "456"
输出: "56088"

 

提示:

  • 1 <= num1.length, num2.length <= 200
  • num1 和 num2 只能由数字组成。
  • num1 和 num2 都不包含任何前导零,除了数字0本身。

解法

方法一:数学乘法模拟

假设 $num1$$num2$ 的长度分别为 $m$$n$,则它们的乘积的长度最多为 $m + n$

证明如下:

  • 如果 $num1$$num2$ 都取最小值,那么它们的乘积为 ${10}^{m - 1} \times {10}^{n - 1} = {10}^{m + n - 2}$,长度为 $m + n - 1$
  • 如果 $num1$$num2$ 都取最大值,那么它们的乘积为 $({10}^m - 1) \times ({10}^n - 1) = {10}^{m + n} - {10}^m - {10}^n + 1$,长度为 $m + n$

因此,我们可以申请一个长度为 $m + n$ 的数组,用于存储乘积的每一位。

从低位到高位,依次计算乘积的每一位,最后将数组转换为字符串即可。

注意判断最高位是否为 $0$,如果是,则去掉。

时间复杂度 $O(m \times n)$,空间复杂度 $O(m + n)$。其中 $m$$n$ 分别为 $num1$$num2$ 的长度。

Python3

class Solution:
    def multiply(self, num1: str, num2: str) -> str:
        if num1 == "0" or num2 == "0":
            return "0"
        m, n = len(num1), len(num2)
        arr = [0] * (m + n)
        for i in range(m - 1, -1, -1):
            a = int(num1[i])
            for j in range(n - 1, -1, -1):
                b = int(num2[j])
                arr[i + j + 1] += a * b
        for i in range(m + n - 1, 0, -1):
            arr[i - 1] += arr[i] // 10
            arr[i] %= 10
        i = 0 if arr[0] else 1
        return "".join(str(x) for x in arr[i:])

Java

class Solution {
    public String multiply(String num1, String num2) {
        if ("0".equals(num1) || "0".equals(num2)) {
            return "0";
        }
        int m = num1.length(), n = num2.length();
        int[] arr = new int[m + n];
        for (int i = m - 1; i >= 0; --i) {
            int a = num1.charAt(i) - '0';
            for (int j = n - 1; j >= 0; --j) {
                int b = num2.charAt(j) - '0';
                arr[i + j + 1] += a * b;
            }
        }
        for (int i = arr.length - 1; i > 0; --i) {
            arr[i - 1] += arr[i] / 10;
            arr[i] %= 10;
        }
        int i = arr[0] == 0 ? 1 : 0;
        StringBuilder ans = new StringBuilder();
        for (; i < arr.length; ++i) {
            ans.append(arr[i]);
        }
        return ans.toString();
    }
}

C++

class Solution {
public:
    string multiply(string num1, string num2) {
        if (num1 == "0" || num2 == "0") {
            return "0";
        }
        int m = num1.size(), n = num2.size();
        vector<int> arr(m + n);
        for (int i = m - 1; i >= 0; --i) {
            int a = num1[i] - '0';
            for (int j = n - 1; j >= 0; --j) {
                int b = num2[j] - '0';
                arr[i + j + 1] += a * b;
            }
        }
        for (int i = arr.size() - 1; i; --i) {
            arr[i - 1] += arr[i] / 10;
            arr[i] %= 10;
        }
        int i = arr[0] ? 0 : 1;
        string ans;
        for (; i < arr.size(); ++i) {
            ans += '0' + arr[i];
        }
        return ans;
    }
};

Go

func multiply(num1 string, num2 string) string {
	if num1 == "0" || num2 == "0" {
		return "0"
	}
	m, n := len(num1), len(num2)
	arr := make([]int, m+n)
	for i := m - 1; i >= 0; i-- {
		a := int(num1[i] - '0')
		for j := n - 1; j >= 0; j-- {
			b := int(num2[j] - '0')
			arr[i+j+1] += a * b
		}
	}
	for i := len(arr) - 1; i > 0; i-- {
		arr[i-1] += arr[i] / 10
		arr[i] %= 10
	}
	i := 0
	if arr[0] == 0 {
		i = 1
	}
	ans := []byte{}
	for ; i < len(arr); i++ {
		ans = append(ans, byte('0'+arr[i]))
	}
	return string(ans)
}

TypeScript

function multiply(num1: string, num2: string): string {
    if (num1 === '0' || num2 === '0') {
        return '0';
    }
    const m: number = num1.length;
    const n: number = num2.length;
    const arr: number[] = Array(m + n).fill(0);
    for (let i: number = m - 1; i >= 0; i--) {
        const a: number = +num1[i];
        for (let j: number = n - 1; j >= 0; j--) {
            const b: number = +num2[j];
            arr[i + j + 1] += a * b;
        }
    }
    for (let i: number = arr.length - 1; i > 0; i--) {
        arr[i - 1] += Math.floor(arr[i] / 10);
        arr[i] %= 10;
    }
    let i: number = 0;
    while (i < arr.length && arr[i] === 0) {
        i++;
    }
    return arr.slice(i).join('');
}

Rust

impl Solution {
    pub fn multiply(num1: String, num2: String) -> String {
        if num1 == "0" || num2 == "0" {
            return String::from("0");
        }
        let (num1, num2) = (num1.as_bytes(), num2.as_bytes());
        let (n, m) = (num1.len(), num2.len());
        let mut res = vec![];
        for i in 0..n {
            let a = num1[n - i - 1] - b'0';
            let mut sum = 0;
            let mut j = 0;
            while j < m || sum != 0 {
                if i + j == res.len() {
                    res.push(0);
                }
                let b = num2.get(m - j - 1).unwrap_or(&b'0') - b'0';
                sum += a * b + res[i + j];
                res[i + j] = sum % 10;
                sum /= 10;
                j += 1;
            }
        }
        res.into_iter()
            .rev()
            .map(|v| char::from(v + b'0'))
            .collect()
    }
}

C#

public class Solution {
    public string Multiply(string num1, string num2) {
        if (num1 == "0" || num2 == "0") {
            return "0";
        }

        int m = num1.Length;
        int n = num2.Length;
        int[] arr = new int[m + n];

        for (int i = m - 1; i >= 0; i--) {
            int a = num1[i] - '0';
            for (int j = n - 1; j >= 0; j--) {
                int b = num2[j] - '0';
                arr[i + j + 1] += a * b;
            }
        }

        for (int i = arr.Length - 1; i > 0; i--) {
            arr[i - 1] += arr[i] / 10;
            arr[i] %= 10;
        }

        int index = 0;
        while (index < arr.Length && arr[index] == 0) {
            index++;
        }

        StringBuilder ans = new StringBuilder();
        for (; index < arr.Length; index++) {
            ans.Append(arr[index]);
        }

        return ans.ToString();
    }
}

PHP

class Solution {
    /**
     * @param string $num1
     * @param string $num2
     * @return string
     */

    function multiply($num1, $num2) {
        $length1 = strlen($num1);
        $length2 = strlen($num2);
        $product = array_fill(0, $length1 + $length2, 0);

        for ($i = $length1 - 1; $i >= 0; $i--) {
            for ($j = $length2 - 1; $j >= 0; $j--) {
                $digit1 = intval($num1[$i]);
                $digit2 = intval($num2[$j]);

                $temp = $digit1 * $digit2 + $product[$i + $j + 1];
                $product[$i + $j + 1] = $temp % 10;

                $carry = intval($temp / 10);
                $product[$i + $j] += $carry;
            }
        }
        $result = implode('', $product);
        $result = ltrim($result, '0');
        return $result === '' ? '0' : $result;
    }
}