## Saturday, January 7, 2012

### Find the starting index of a sub-array from the first array

```
/**
* Please note: even though I just used Integer Array in this task, the algorithm could be
* used for any Array
*/
public static int firstIndexOfSubArray(int[] arr1, int[] arr2)
{
if (arr1 == null || arr2 == null)
{
return -1;
}
int len1 = arr1.length;
int len2 = arr2.length;
if (len1 == 0 || len2 == 0 || len1 < len2)
{
return -1;
}
//start the sliding window from the left side
int slidingFrom = 0;
int i = slidingFrom;
int j = 0;
int firstEqualIndex = -1;
while (true)
{
//if the sliding window is out of boundary,just return
if (len2 > (len1 - slidingFrom))
{
return -1;
}
//find the first equal element
if (firstEqualIndex == -1)
{
if (arr1[i] == arr2)
{
firstEqualIndex = i;
slidingFrom = i;
j = 1;
}
i++;
continue;
}
//if already found the first equal element, compare the following elements
if (arr1[i] == arr2[j])
{
//if equal, contiue to compare until the last element in array 2
i++;
j++;
if (j == len2)
{
break;
}
}
//if not equal, slide to the next one and compare every element in array 2 again
else
{
firstEqualIndex = -1;
i = slidingFrom + 1;
}
}
return firstEqualIndex;
}
```