Created
July 27, 2016 20:39
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longest common substring - brute force solution O(n^3) - first writing, 2 bugs, need the debugger; static analysis catching one bug only
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using System; | |
using System.Collections.Generic; | |
using System.Linq; | |
using System.Text; | |
using System.Threading.Tasks; | |
namespace longestCommonSubstring | |
{ | |
class Program | |
{ | |
static void Main(string[] args) | |
{ | |
string test = longestCommonSubstring("abc123def","hij123klmn"); | |
} | |
/* | |
* July 27, 2016 | |
* | |
* Design: | |
* Using the start position as a variable to do brute force search, | |
* Two strings - common substring | |
* iterate one string on start position, | |
* iterate second string on start position as well | |
* | |
* Time complexity - O(n^4) -> O(n^3) | |
* instead of two variables - start position and end position - O(n^4) | |
* | |
* Read C++ code fist to get the idea, | |
* | |
* and then, write C# code | |
* | |
* Target: try to finish it in 20 minutes in first writing. | |
*/ | |
public static string longestCommonSubstring(string s1, string s2) | |
{ | |
if (s1 == null || s1.Length == 0 || s2 == null || s2.Length == 0) | |
return string.Empty; | |
int len1 = s1.Length; | |
int len2 = s2.Length; | |
int maximum = 0; | |
string res = string.Empty; | |
for(int i=0; i<len1; i++ ) | |
for (int j = 0; j < len2; j++) | |
{ | |
int start1 = i; | |
int start2 = j; | |
int cur = 0; | |
while (start1 < len1 && start2 < len2) | |
{ | |
char c1 = s1[start1]; | |
char c2 = s2[start2]; | |
if (c1 == c2) | |
{ | |
start1++; | |
start2++; | |
cur++; | |
} | |
else | |
{ | |
if (cur > maximum) | |
{ | |
//cur = maximum; // bug001 - opposite | |
maximum = cur; | |
int lastPos = start1 - 1; // s1 - start: i, end: lastPos | |
res = s1.Substring(i, lastPos - i + 1); //no index out-of-range error | |
} | |
break; // bug002 - forget to add break! first writing, otherwise, deadloop! | |
} | |
} | |
} | |
return res; | |
} | |
} | |
} |
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