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  1. // C++ program to multiply two numbers 
  2. // represented as strings. 
  3. #include <bits/stdc++.h> 
  4. using namespace std; 
  5.  
  6. // Multiplies str1 and str2, and prints result. 
  7. string multiply(string num1, string num2) 
  8. { 
  9. 	int n1 = num1.size(); 
  10. 	int n2 = num2.size(); 
  11. 	if (n1 == 0 || n2 == 0) 
  12. 		return "0"; 
  13.  
  14. 	// will keep the result number in vector 
  15. 	// in reverse order 
  16. 	vector<int> result(n1 + n2, 0); 
  17.  
  18. 	// Below two indexes are used to find positions 
  19. 	// in result. 
  20. 	int i_n1 = 0; 
  21. 	int i_n2 = 0; 
  22.  
  23. 	// Go from right to left in num1 
  24. 	for (int i = n1 - 1; i >= 0; i--) { 
  25. 		int carry = 0; 
  26. 		int n1 = num1[i] - '0'; 
  27.  
  28. 		// To shift position to left after every 
  29. 		// multiplication of a digit in num2 
  30. 		i_n2 = 0; 
  31.  
  32. 		// Go from right to left in num2 
  33. 		for (int j = n2 - 1; j >= 0; j--) { 
  34. 			// Take current digit of second number 
  35. 			int n2 = num2[j] - '0'; 
  36.  
  37. 			// Multiply with current digit of first number 
  38. 			// and add result to previously stored result 
  39. 			// at current position. 
  40. 			int sum = n1 * n2 + result[i_n1 + i_n2] + carry; 
  41.  
  42. 			// Carry for next iteration 
  43. 			carry = sum / 10; 
  44.  
  45. 			// Store result 
  46. 			result[i_n1 + i_n2] = sum % 10; 
  47.  
  48. 			i_n2++; 
  49. 		} 
  50.  
  51. 		// store carry in next cell 
  52. 		if (carry > 0) 
  53. 			result[i_n1 + i_n2] += carry; 
  54.  
  55. 		// To shift position to left after every 
  56. 		// multiplication of a digit in num1. 
  57. 		i_n1++; 
  58. 	} 
  59.  
  60. 	// ignore '0's from the right 
  61. 	int i = result.size() - 1; 
  62. 	while (i >= 0 && result[i] == 0) 
  63. 		i--; 
  64.  
  65. 	// If all were '0's - means either both or 
  66. 	// one of num1 or num2 were '0' 
  67. 	if (i == -1) 
  68. 		return "0"; 
  69.  
  70. 	// generate the result string 
  71. 	string s = ""; 
  72. 	while (i >= 0) 
  73. 		s += std::to_string(result[i--]); 
  74.  
  75. 	return s; 
  76. } 
  77.  
  78. // Driver code 
  79. int main() 
  80. { 
  81. 	string str1 = "454545454545454545"; 
  82.  
  83. 	cout << multiply(str1, str1); 
  84.  
  85. 	return 0; 
  86. } 
  87.  
Success #stdin #stdout 0s 4268KB
comments (?)
stdin 
Standard input is empty
stdout 
206611570247933883884297520661157025
https://ideone.com/mV4ylo
language:
C++14 (gcc 8.3)
created:
5 years ago
visibility:
public

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