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# 1796. Second Largest Digit in a String

## LeetCode [1796. Second Largest Digit in a String](https://leetcode-cn.com/problems/second-largest-digit-in-a-string/)

### Description

Given an alphanumeric string `s`, return *the **second largest** numerical digit that appears in* `s` *, or*`-1` *if it does not exist*.

An **alphanumeric** *\*\** string is a string consisting of lowercase English letters and digits.

**Example 1:**

```
Input: s = "dfa12321afd"
Output: 2
Explanation: The digits that appear in s are [1, 2, 3]. The second largest digit is 2.
```

**Example 2:**

```
Input: s = "abc1111"
Output: -1
Explanation: The digits that appear in s are [1]. There is no second largest digit. 
```

**Constraints:**

* `1 <= s.length <= 500`
* `s` consists of only lowercase English letters and/or digits.

### Tags

String

### Solution

This problem is similar to "find the second largest number", **but requires the largest number is not equal to the second largest one**. We initialize both the largest number `first` and the 2nd largest number `second` to -1. Iterave over the string and we only consider the case that the current character is a numeric byte/rune. We convert it from byte/rune to int type variable `n`.

* If `n > first`, we pass the value of `first` to `second`, then assign `n` to `first`;
* If `n > second && n != first`, we only update the value of `second` with `n`.

Finally, we return `second`.

### Complexity

* Time complexity: $$O(n)$$
* Space complexity: $$O(1)$$

### Code

```go
func secondHighest(s string) int {
	first, second := -1, -1
	for _, c := range s {
		if c >= '0' && c <= '9' {
			if n := int(c - '0'); n > first {
				second = first
				first = n
			} else if n > second && n != first {
				second = n
			}
		}
	}
	return second
}
```