Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.
Example 1:
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Input: num1 = "2", num2 = "3"
Output: "6"
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Example 2:
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Input: num1 = "123", num2 = "456"
Output: "56088"
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Note:
- The length of both
num1 and num2 is < 110.
- Both
num1 and num2 contain only digits 0-9.
- Both
num1 and num2 do not contain any leading zero, except the number 0 itself.
- You must not use any built-in BigInteger library or convert the inputs to integer directly.
解法
其实就是大数相乘的算法，这里需要注意的是在每次做乘法运算时，不考虑进位问题，直接把结果存入结果数组的对应位上，最后统一处理进位问题。注意，计算从数组的尾部开始。
具体代码如下：
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class {
public String multiply(String num1, String num2) {
int l1 = num1.length();
int l2 = num2.length();
int[] result = new int[l1 + l2];
int k = l1 + l2 - 1;
for (int j = l2 - 1; j >= 0; j--) {
int kk = k;
for (int i = l1 - 1; i >= 0; i--) {
result[kk] += (num2.charAt(j) - '0') * (num1.charAt(i) - '0');
kk--;
}
k--;
}
for (k = l1 + l2 - 1; k >= 0; k--) {
int j = result[k] / 10;
result[k] = result[k] % 10;
if (k >= 1) {
result[k - 1] += j;
}
}
StringBuffer s = new StringBuffer();
int i = 0;
for (; i < result.length && result[i] == 0; i++) ;
for (; i < result.length; i++) {
s.append(result[i]);
}
if (s.length() == 0) {
return "0";
}
String res = new String(s);
return res;
}
}
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