LeetCode第五题：给定一个字符串 s，找到 s 中最长的回文子串。你可以假设 s 的最大长度为 1000。
示例 1：
输入: "babad"
输出: "bab"
注意: "aba" 也是一个有效答案。

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSString* s = @"bcaddad";
        NSMutableArray* marray = [NSMutableArray array];
        for (int i = 0; i < s.length; i++) {//获取收尾同字母字符串
            NSString* iletter = [s substringWithRange:NSMakeRange(i, 1)];
            for (int m = i+1; m<s.length; m++) {
                NSString* letter = [s substringWithRange:NSMakeRange(m, 1)];
                if ([iletter isEqualToString:letter]) {
                    NSString* substring = [s substringWithRange:NSMakeRange(i, m-i+1)];
                    [marray addObject:substring];
                }
            }
        }
        NSMutableArray* marray_2 = [marray mutableCopy];
        for (NSString* string in marray) {//筛选出回文字符串
            if (string.length%2 == 0) {//偶数字符串
                for (int i = 1; i < string.length/2; i++) {
                    NSString* s_1 = [string substringWithRange:NSMakeRange(i, 1)];
                    NSString* s_2 = [string substringWithRange:NSMakeRange(string.length-1-i, 1)];
                    NSLog(@"s1-s2---%@-%@",s_1,s_2);
                    if (![s_1 isEqualToString:s_2]) {
                        [marray_2 removeObject:string];
                    }
                }
            }
            else {//奇数字符串
                for (int i = 1; i < string.length+1/2; i++) {
                    NSString* s_1 = [string substringWithRange:NSMakeRange(i, 1)];
                    NSString* s_2 = [string substringWithRange:NSMakeRange(string.length-1-i, 1)];
                    NSLog(@"s1-s2---%@-%@",s_1,s_2);
                    if (![s_1 isEqualToString:s_2]) {
                    
                        [marray_2 removeObject:string];
                    }
                }
                
            }
            
            
        }
        NSLog(@"首尾同字母字符串---%@",marray);
        NSLog(@"回文字符串---%@",marray_2);
        NSString* maxString = @"";
        for (NSString* string in marray_2) {//将最长字符串赋值给max
            if (string.length > maxString.length) {
                maxString = string;
            }
        }
        
        NSLog(@"最长回文字符串---%@",maxString);
    }
    return 0;
}