算法练习（8）：牛客在线编程08 字符串

package jz.bm;

import java.util.Arrays;

public class bm8 {
    /**
     * BM83 字符串变形
     */
    public String trans(String s, int n) {
        StringBuilder res = new StringBuilder();
        //大小写转换
        for (int i = 0; i < n; i++) {
            if (s.charAt(i) >= 'a' && s.charAt(i) <= 'z') {
                res.append(Character.toUpperCase(s.charAt(i)));
            } else if (s.charAt(i) >= 'A' && s.charAt(i) <= 'Z') {
                res.append(Character.toLowerCase(s.charAt(i)));
            } else {
                res.append(s.charAt(i));
            }
        }
        //反转顺序
        res = res.reverse();
        //单词顺序转换
        for (int i = 0; i < n; i++) {
            if (res.charAt(i) != ' ') {
                int j = i; //每个单词的起点
                while (j < n && res.charAt(j) != ' ') {
                    j++;
                }
                StringBuilder stringBuilder = new StringBuilder(res.substring(i, j));
                res.replace(i, j, stringBuilder.reverse().toString());
                i = j;
            }
        }
        return res.toString();
    }
    /**
     * BM84 最长公共前缀
     */
    public String longestCommonPrefix (String[] strs) {
        if (strs.length == 0) {
            return "";
        }
        StringBuilder res = new StringBuilder();
        for (int i = 0; i < strs[0].length(); i++) {
            char c = strs[0].charAt(i);
            for (int j = 1; j < strs.length; j++) {
                if (i >= strs[j].length() || strs[j].charAt(i) != c) {
                    return res.toString();
                }
            }
            res.append(c);
        }
        return res.toString();
    }
    /**
     * BM85 验证IP地址
     */
    public String solve (String IP) {
        if (ipv4(IP)) {
            return "IPv4";
        } else if (ipv6(IP)) {
            return "IPv6";
        } else {
            return "Neither";
        }
    }

    private boolean ipv4(String IP) {
        String[] strings = IP.split("\\.");
        //防止尾部是.
        if (strings.length != 4 || IP.endsWith(".")) {
            return false;
        }
        for (int i = 0; i < 4; i++) {
            if (strings[i].length() == 0) {
                return false;
            }
            for (int j = 0; j < strings[i].length(); j++) {
                //开头不能为0
                if (j == 0 && strings[i].charAt(j) == '0') {
                    return false;
                }
                //10进制
                if (strings[i].charAt(j) > '9' || strings[i].charAt(j) < '0') {
                    return false;
                }
            }
            //0-255
            int num = Integer.parseInt(strings[i]);
            if (num < 0 || num > 255) {
                return false;
            }
        }
        return true;
    }

    private boolean ipv6(String IP) {
        String[] strings = IP.split("\\:");
        //防止尾部是:
        if (strings.length != 8 || IP.endsWith(":")) {
            return false;
        }
        for (int i = 0; i < 8; i++) {
            //不为null,没有多余的0
            if (strings[i].length() == 0 || strings[i].length() > 4 || strings[i].startsWith("00")) {
                return false;
            }
            for (int j = 0; j < strings[i].length(); j++) {
                //16进制
                if (!((strings[i].charAt(j) >= '0' && strings[i].charAt(j) <= '9')
                || (strings[i].charAt(j) >= 'a' && strings[i].charAt(j) <= 'f')
                || (strings[i].charAt(j) >= 'A' && strings[i].charAt(j) <= 'F'))) {
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * BM86 大数加法
     */
    public String solve (String s, String t) {
        if (s == null || s.equals("")) {
            return t;
        }
        if (t == null || t.equals("")) {
            return s;
        }
        int i = s.length() - 1, j = t.length() - 1;
        int carry = 0;
        StringBuilder res = new StringBuilder();
        while (i >= 0 || j >= 0) {
            int a = i < 0 ? 0 : s.charAt(i) - '0';
            int b = j < 0 ? 0 : t.charAt(j) - '0';
            res.append((a + b + carry) % 10);
            carry = (a + b + carry) / 10;
            i--;
            j--;
        }
        if (carry != 0) {
            res.append(carry);
        }
        return res.reverse().toString();
    }
}