最长回文字符串

回文串：正反读都是一样的字符串，如：abba，aba,
给你一个字符串 s，找到 s 中最长的回文子串。

- (void)test {
  NSDictionary *dic = [self searchMaxLengthPalindrome:@"abacdcaba"];
  NSLog(@"%@", dic);
}

- (NSDictionary *)searchMaxLengthPalindrome:(NSString *)str {
  NSInteger maxlength = 0, leftStart = 0;
  for (NSInteger i = 0; i < str.length; i++) {
    NSInteger left = i - 1, right = i + 1, length = 1;
    
    char iC = [str characterAtIndex:i];
    while (left >= 0 && [str characterAtIndex:left] == iC) {
      left--;
      length++;
    }
    while (right < str.length && [str characterAtIndex:right] == iC) {
      right++;
      length++;
    }
    while (left >= 0 && right < str.length && [str characterAtIndex:left] == [str characterAtIndex:right]) {
      left--;
      right++;
      length += 2;
    }
    if (maxlength < length) {
      maxlength = length;
      leftStart = left;
    }
  }
  return
  @{
    @"left": @(leftStart + 1),
    @"right": @(leftStart + maxlength),
    @"lenght": @(maxlength),
    @"value": [[str substringToIndex:(leftStart + 1 + maxlength)] substringFromIndex:leftStart + 1]
  };
}

输出的结果：
{
left = 0;
lenght = 9;
right = 9;
value = abacdcaba;
}