1. 示例 1：
    2. 输入：s = "babad"
    3. 输出："bab"
    4. 解释："aba" 同样是符合题意的答案。
    6. 示例 2：
    7. 输入：s = "cbbd"
    8. 输出："bb"
    10. 示例 3：
    11. 输入：s = "a"
    12. 输出："a"
    14. 示例 4：
    15. 输入：s = "ac"
    16. 输出："a"
    1. -(NSString *)maxString:(NSString *)str{
    2.     if (str.length <= 1) {
    3.         return str;
    4.     }
    5.     NSString *tempJi;
    6.     NSString *tempOu;
    7.     NSString *resultStr = [str substringToIndex:1];
    8.     //i:1循环次数，2关键字索引
    9.     for (int i = 0 ; i< str.length; i++) {
    10.         //内循环mac次数
    11.         int loopCount = i < str.length-1-i ? i : (int)(str.length-1-i);
    13.         //奇数
    14.         if (loopCount != 0) {
    15.             for (int j = 1; j< loopCount+1; j++) {
    17.                 NSString *leftc = [str substringWithRange:NSMakeRange(i-j, 1)];
    18.                 NSString *rightc = [str substringWithRange:NSMakeRange(i+j, 1)];
    20.                 if ([leftc isEqualToString:rightc] ) {
    21.                     tempJi = [str substringWithRange:NSMakeRange(i-j, 2*j+1)];
    22.                 }else{
    23.                     break;
    24.                 }
    25.             }
    26.         }
    27.         resultStr = tempJi.length > resultStr.length ? tempJi : resultStr;
    29.         //偶数 add ddac
    30.         loopCount = i < str.length-1-i ? i : (int)(str.length-1-i);
    31.         if (loopCount != 0) {
    32.             for (int j = 0; j< loopCount+1; j++) {
    34.                 NSString *leftc = [str substringWithRange:NSMakeRange(i-j, 1)];
    35.                 NSString *rightc;
    36.                 if (i+j+1 < str.length) {
    37.                     rightc = [str substringWithRange:NSMakeRange(i+j+1, 1)];
    38.                 }
    40.                 if ([leftc isEqualToString:rightc]) {
    41.                     tempOu = [str substringWithRange:NSMakeRange(i-j, 2*(j+1))];
    42.                 }else{
    43.                     break;
    44.                 }
    45.             }
    46.         }
    47.         resultStr = tempOu.length > resultStr.length ? tempOu : resultStr;
    48.     }
    49.     return  resultStr;
    50. }
    52. NSLog(@"结果%@",[self maxString:@"caaaac"]);