主要方法：

• 獲取子字符串：substringWithRange:
• 替換字符串：stringByReplacingCharactersInRange:range

for (int i = 0; i<[string length]; i++) {
    //截取字符串中的每一個字符
    NSString *s = [string substringWithRange:NSMakeRange(i, 1)];
    NSLog(@"string is %@",s);
    if ([s isEqualToString:@"m"]) {
        NSRange range = NSMakeRange(i, 1);
        //將字符串中的“m”轉(zhuǎn)化為“w”
        string =   [string stringByReplacingCharactersInRange:range withString:@"w"];
        
    }
}
NSLog(@"%@",string);