目的：在任意的字符串中求出最长的回文字符串

 

思路：（适用于任何语言）

 

1、判断当前给定的字符串是否是相同的字符串（也就是所有字符都相同），如果是直接返回了。

 

2、如果第一步没有返回，就以非第一个字符为轴，分别求出以它为轴的，双数回文字符串，和单数回文字符串的长度。

 

 3、把上述求出来的长度，和已经有的回文字符串长度对比，如果长于已经存在的回文字符串的长度，就进行赋值。

 

4、判断最后统计的回文字符串如果等于空，并且给出的字符大于0，就把第一个字符赋值给最长回文字符串，并且返回。

 

代码如下：

public class Test {

    public String aa(String s){

        char[] arr = s.toCharArray();               //把字符串转换成字符
        String max = "";                            //收集最长的回文字符串
        int k = 0;                                  //统计当前 最长回文字符串的长度 / 同时统计以当前字符串为轴的时候 双数回文字符串的长度
        int flag = 0;                               //0 表示 以该字符为轴的是 双数回文字符串
        int m = 0;                                  //
        int p = 0;                                  //统计以当前字符串为轴的时候 单数回文字符串的长度
        //如果字符串全部是一种字符的话  直接返回
        for(k = 0;k < arr.length-1; k++){
            if (arr[k] != arr[k+1])
                break;
        }
        if(k == arr.length -1)
            return s;
        //以非第一个字符为轴 ，来统计以它为轴   双数回文字符串 和 单数 回文字符串的长度   并且把最长的回文字符串 赋值给 max
        for(int i = 1;i < arr.length; i++){
            k = 0;
            p = 0;
            for (flag = 0;flag <= 1; flag++)
                for(int j = i-1;j >= 0; j--){
                    if ( flag == 1 && j == i-1 )
                        m = 0;
                    if(flag == 0){  //双数
                        if(i+k < arr.length)
                            if(arr[j] == arr[i+k]){
                                k++;
                            }else {
                                break;
                            }
                    }
                    else {  //单数

                        if(j >= 0 && i+m+1 < arr.length )
                            if(arr[j] == arr[i+1+(m++)]){
                                p++;
                            }else {
                                break;
                            }
                    }
                }

            if ( p >= k ){  // k 表示双  p表示单
                k = p;
                flag = 1;
            } else
                flag = 0;

            if(flag == 0){
                if(max.length() < 2*k){
                    max = "";
                    for(int j = i-k;j < i+k; j++){
                        max += arr[j];
                    }
                }
            } else{
                if(max.length() < 2*k+1){
                    if(i - k >= 0 && i+k <= s.length()-1){
                        max = "";
                        for(int j = i-k;j <= i+k; j++){
                            max += arr[j];
                        }
                    }
                }
            }
        }
        if (max.equals("") && arr.length > 0)   //当输入空串和没有任何长度大于2的回转字符做的特殊处理
            max += arr[0];

        return max;
    }

    public static void main(String[] args) {
        Test t = new Test();
        //测试数据
        System.out.println(t.aa("lqlvciwekzxapmjxyddlaoqhfhwphamsyfwjinkfvciucjhdgwodvmnpkibumexvlsxxumvrznuuyqfavmgwfnsvfbrvqhlvhpxaqehsiwxzlvvtxsnmlilbnmvnxyxitxwoahjricdjdncvartepfpdfndxqoozkfpdmlpvshzzffsspdjzlhmamqooooor"));
        System.out.println(t.aa("oooooor"));
        System.out.println(t.aa("aba"));
        System.out.println(t.aa("abb"));
        System.out.println(t.aa("bba"));
        System.out.println(t.aa("aaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggggggggghhhhhhhhhhiiiiiiiiiijjjjjjjjjjkkkkkkkkkkllllllllllmmmmmmmmmmnnnnnnnnnnooooooooooppppppppppqqqqqqqqqqrrrrrrrrrrssssssssssttttttttttuuuuuuuuuuvvvvvvvvvvwwwwwwwwwwxxxxxxxxxxyyyyyyyyyyzzzzzzzzzzyyyyyyyyyyxxxxxxxxxxwwwwwwwwwwvvvvvvvvvvuuuuuuuuuuttttttttttssssssssssrrrrrrrrrrqqqqqqqqqqppppppppppoooooooooonnnnnnnnnnmmmmmmmmmmllllllllllkkkkkkkkkkjjjjjjjjjjiiiiiiiiiihhhhhhhhhhggggggggggffffffffffeeeeeeeeeeddddddddddccccccccccbbbbbbbbbbaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggggggggghhhhhhhhhhiiiiiiiiiijjjjjjjjjjkkkkkkkkkkllllllllllmmmmmmmmmmnnnnnnnnnnooooooooooppppppppppqqqqqqqqqqrrrrrrrrrrssssssssssttttttttttuuuuuuuuuuvvvvvvvvvvwwwwwwwwwwxxxxxxxxxxyyyyyyyyyyzzzzzzzzzzyyyyyyyyyyxxxxxxxxxxwwwwwwwwwwvvvvvvvvvvuuuuuuuuuuttttttttttssssssssssrrrrrrrrrrqqqqqqqqqqppppppppppoooooooooonnnnnnnnnnmmmmmmmmmmllllllllllkkkkkkkkkkjjjjjjjjjjiiiiiiiiiihhhhhhhhhhggggggggggffffffffffeeeeeeeeeeddddddddddccccccccccbbbbbbbbbbaaaa"));
        System.out.println(t.aa("cbbd"));
        System.out.println(t.aa("fttttf"));
        System.out.println(t.aa("eabcb"));
        System.out.println(t.aa("aaabaaa"));
        System.out.println(t.aa("aaabaaaaaa"));
        System.out.println(t.aa("ac"));
        System.out.println(t.aa(""));
        System.out.println(t.aa("sooos"));
        System.out.println(t.aa("azwdzwmwcqzgcobeeiphemqbjtxzwkhiqpbrprocbppbxrnsxnwgikiaqutwpftbiinlnpyqstkiqzbggcsdzzjbrkfmhgtnbujzszxsycmvipjtktpebaafycngqasbbhxaeawwmkjcziybxowkaibqnndcjbsoehtamhspnidjylyisiaewmypfyiqtwlmejkpzlieolfdjnxntonnzfgcqlcfpoxcwqctalwrgwhvqvtrpwemxhirpgizjffqgntsmvzldpjfijdncexbwtxnmbnoykxshkqbounzrewkpqjxocvaufnhunsmsazgibxedtopnccriwcfzeomsrrangufkjfzipkmwfbmkarnyyrgdsooosgqlkzvorrrsaveuoxjeajvbdpgxlcrtqomliphnlehgrzgwujogxteyulphhuhwyoyvcxqatfkboahfqhjgujcaapoyqtsdqfwnijlkknuralezqmcryvkankszmzpgqutojoyzsnyfwsyeqqzrlhzbc"));
    }

}