# Kotlin Algorithm Challenge #2

Alex Woods

Jan 02, 2020

The following problem is the Longest Substring Without Repeating Characters problem on LeetCode.

# Problem

Given a string, find the length of the longest substring without repeating characters.

```
# Example 1
Input: "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.
# Example 2
Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.
# Example 3
Input: "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
```

There’s one nice example that comes up in the submissions that highlights an edge case as well, which is:

```
Input: "abba"
Output: 2
```

# Solution

When we’re iterating through the array, and evaluating whether a new substring should be the max substring, we need to know two things:

- the length of the current max substring
- what characters are included in the current substring

Once we have those two things, it’s just a matter of iterating through the array once, and it’s not so complicated.

Getting the current max is simple, it’s just a matter of maintaining a max, initialized at 0, and update it as we go through the array.

## Getting the current substring

This part is a little bit more complicated.

```
abcabcbb
^ ^
i j
```

Say we’re iterating through the above string. We’re at position *j*. The current substring we’re looking at is everything up until *i*.

If we can easily find *i*, then the length of the current substring is simply *j - i*. But how do we find *i*?

Speaking generally, *i* is the last occurrence of *any* char in our current substring.

We can break that into two cases:

*i*is the index of the last occurrence of the char we are currently sitting at, as we iterate through.*i*is the index of the last occurrence of any other char in the current substring

I’ve renamed `i`

to `lastRepeat`

, because what it actually represents is the last repeat of *any* char in the current substring.

Here is the final code:

```
fun maxSubstring(str: String): Int {
val map: HashMap<Char, Int> = hashMapOf()
var max = 0
var lastRepeat = -1
str.forEachIndexed { j, value ->
val lastOccurence = map.get(value)
if (lastOccurence != null) {
lastRepeat = maxOf(lastRepeat, lastOccurence)
}
max = maxOf(max, j - lastRepeat)
map.put(value, j)
}
return max
}
```

# Complexity

We’re iterating through the array once, so the time complexity is `O(n)`

.

The space usage is constant, because while we do have a hashmap, the maximum number of keys is limited by the total number of ASCII characters.

One final note:

It turns out, `forEachIndexed`

is less efficient than a simple for loop, e.g.

```
import kotlin.system.measureNanoTime
val x = (1..10000).toList().toTypedArray()
fun foo(): Unit {}
measureNanoTime { x.forEachIndexed { _, _ -> foo() } } // 674349
measureNanoTime { for (temp in 0 until x.size) { foo() } } // 192791
```

Note that we’re talking in nanoseconds here—I definitely think the readability gains from Kotlin’s functional APIs are worth a few extra nanoseconds.