---
title: 'LeetCode 917. Reverse Only Letters'
disqus: hackmd
---

# LeetCode 917. Reverse Only Letters

## Description

Given a string s, reverse the string according to the following rules:

All the characters that are not English letters remain in the same position.
All the English letters (lowercase or uppercase) should be reversed.
Return s after reversing it.

## Example

Input: s = "ab-cd"
Output: "dc-ba"

Input: s = "a-bC-dEf-ghIj"
Output: "j-Ih-gfE-dCba"

## Constraints

1 <= s.length <= 100
s consists of characters with ASCII values in the range [33, 122].
s does not contain '\"' or '\\'.

## Answer

此題只轉英文字，所以先抓頭尾，設while(head<tail)，然後用while將非英文的字元跳過，用^*^head!='\0'和S < tail當邊界來考慮全跳過的情況，然後再次檢查head<tail，確認之後才反轉。

//2021_11_18
char * reverseOnlyLetters(char * S) {
    int len = strlen(S);
    char *head = S, *tail = S + len - 1, temp = '\0';
    while(head < tail){
        while(*head != '\0' && (*head < 'A' || (*head > 'Z' && *head < 'a') || *head > 'z')){head++;}
        while(S < tail && (*tail < 'A' || (*tail > 'Z' && *tail < 'a') || *tail > 'z')){tail--;}
        if(head < tail){
            temp = *head;
            *head = *tail;
            *tail = temp;
            head++;
            tail--;
        }
    }
    return S;
}

## Link
https://leetcode.com/problems/reverse-only-letters/
###### tags: `Leetcode`