经典算法（四） 数组相关 & 螺旋矩阵 & 数字大小写转换 & 字符串相关

一、求所有子数组的和的最大值

public static void main(String[] args) {
        int[] a = { 1, -2, 3, 10, -4, 7, 2, -5 };
        FindMaxSubAry(a);
    }

    public static void FindMaxSubAry(int[] arr) {
        int maxStartIndex = 0;// 最大数组开始坐标
        int maxEndIndex = 0; // 最大数组结束坐标
        int max = 0; // 最大数组的和
        int tempSum = 0;
        for (int i = 0; i < arr.length; i++) {
            tempSum += arr[i];
            if (tempSum <= 0) {
                tempSum = 0;
                max = 0;
                maxStartIndex = i + 1;
            }
            if (tempSum > max) {
                max = tempSum;
                maxEndIndex = i;
            }
        }
        System.out.println(" sum:" + max);
        print(arr, maxStartIndex, maxEndIndex);
    }

    public static void print(int[] arr, int startIndex, int endIndex) {
        for (int i = startIndex; i <= endIndex; i++) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();
    }

二、顺时针打印矩阵

分析：构造二维矩阵

1     2    3     4    
   5     6    7     8    
   9    10   11   12   
  13  14   15   16  
   按照从外到里以顺时针的顺序依次打印
   打印：1 2 3 4 8 12 16 15 14 13 9 5 6 7 11 10

public static void print(int[][] a,int start,int end){
        if(start>=end||end<0){
            return;
        }
        for(int i=start;i<=end;i++){
            System.out.print(a[start][i]+" ");
        }

        for(int i=start+1;i<=end;i++){
            System.out.print(a[i][end]+" ");
        }
        for(int i=end-1;i>=start;i--){
            System.out.print(a[end][i]+" ");
        }

        for(int i=end-1;i>start;i--){
            System.out.print(a[i][start]+" ");
        }
        print(a, start+1, end-1);
    }

 三、2110980789转化为“二十一亿一千零九十八万零七百八十九”

分析：可以分4位为一个量级。其中0789 量级单位 为空，1098量级单位为 万，21量级单位为亿。这四位字符串在以个十百千的形式计算。

package com.amei.java.algorithm;

import java.util.HashMap;
import java.util.Map;
import java.util.Stack;
/**
 * 将阿拉伯数字转换为大写数字
 * 123010980789转化为“二十一亿一千零九十八万零七百八十九”
 * @param num
 */
public class Common {
    static Map<Integer, String> map = new HashMap<Integer, String>();
    static {
        map.put(1, "");
        map.put(2, "十");
        map.put(3, "百");
        map.put(4, "千");
    }

    public static void main(String[] args) {
        NumberChange(2110980789);
    }

    static String change(int k, char ch) {
        String number = numTobigString(ch);
        String unit = map.get(k);
        if (ch == '0') {
            return number;
        } else {
            return number + unit;
        }
    }

    /**
     * 123010980789转化为“二十一亿一千零九十八万零七百八十九”
     *
     * @param num
     */
    static void NumberChange(int num) {
        String s = num + "";

        char[] ch = s.toCharArray();
        int flag = 0;//控制量级单位
        int k = 1;
        Stack<String> st = new Stack<String>();
         for (int i = ch.length - 1; i >= 0; i--) {
            if (flag == 1 && k == 1) {
                st.add("万");
            }
            if (flag == 2 && k == 1) {
                st.add("亿");
            }
            st.add(change(k, ch[i]));

            if (k % 4 == 0) {
                flag++;
                k = 1;
            } else {
                k++;
            }
        }
        int size = st.size();
        for (int i = 0; i < size; i++) {
            System.out.print(st.pop());
        }
    }

    static String numTobigString(char c) {
        if (c == '1') {
            return "一";
        } else if (c == '2') {
            return "二";
        } else if (c == '3') {
            return "三";
        } else if (c == '4') {
            return "四";
        } else if (c == '5') {
            return "五";
        } else if (c == '6') {
            return "六";
        } else if (c == '7') {
            return "七";
        } else if (c == '8') {
            return "八";
        } else if (c == '9') {
            return "九";
        } else if (c == '0') {
            return "零";
        }
        return null;
    }
}

 四、判断一个ip是否属于某个ip段

分析：将ip转换为数字比较大小，可以看成ip是逢256就进位的数字

public static boolean ipExistsInRange(String ip, String ipSection) {
        ipSection = ipSection.trim();
        ip = ip.trim();
        int idx = ipSection.indexOf('-');
        String beginIP = ipSection.substring(0, idx);
        String endIP = ipSection.substring(idx + 1);
        return getIp2long2(beginIP) <= getIp2long2(ip) && getIp2long2(ip) <= getIp2long2(endIP);
    }

    public static long getIp2long(String ip) {
        ip = ip.trim();
        String[] ips = ip.split("\\.");
        long ip2long = 0L;
        for (int i = 0; i < 4; ++i) {
            //左移8位，相当于乘以256
            ip2long = ip2long << 8 | Integer.parseInt(ips[i]);
        }
        return ip2long;
    }

    public static long getIp2long2(String ip) {
        ip = ip.trim();
        String[] ips = ip.split("\\.");
        long ip1 = Integer.parseInt(ips[0]);
        long ip2 = Integer.parseInt(ips[1]);
        long ip3 = Integer.parseInt(ips[2]);
        long ip4 = Integer.parseInt(ips[3]);
        long ip2long = 1L * ip1 * 256 * 256 * 256 + ip2 * 256 * 256 + ip3 * 256 + ip4;
        return ip2long;
    }

    public static void main(String[] args) {
        String ip = "10.10.10.116";
        String ipSection = "10.10.1.00-10.10.255.255";
        boolean exists = ipExistsInRange(ip, ipSection);
        System.out.println(exists);
    }

 五、将字符串 abcd;3456789; 反转为 dcba;9876543;

public class StrReverse {
    public static void main(String[] args) {
        String str = "abcd;3456789";
        System.out.println(sectionReverse(str));
    }

    static String sectionReverse(String str) {
        char[] c = str.toCharArray();
        int start = ;
        int end = ;
        for (int i = ; i < c.length; i++) {
            if (c[i] == ';') {
                end = i - ;
                reverse(c, start, end);
                start = i + ;
            }else if(i==c.length-){
                end=i;
                reverse(c, start, end);
            }
        }
        return String.valueOf(c);
    }

    static void reverse(char[] c, int start, int end) {
        char temp;
        for (int j = ; j < (end - start) /  + ; j++) {
            temp = c[end - j];
            c[end - j] = c[j + start];
            c[j + start] = temp;
        }
    }
}

 六、统计字符串“abaabbccdfeffsfsgggaabgee”中 出现且仅出现n次的第一个字母

public class StrRepeat {

    public static void main(String[] args) {
        String str = "abaabbccdfeffsfsgggaabgee;";
        System.out.println(repeatCharFisrt(str, 4));
    }

    static char repeatCharFisrt(String str, int repeatNum) {
        Map<Character, Integer> map = new LinkedHashMap<Character, Integer>();
        char[] c = str.toCharArray();
        for (int i = 0; i < c.length; i++) {
            if (map.containsKey(c[i])) {
                map.put(c[i], map.get(c[i]) + 1);
            } else {
                map.put(c[i], 1);
            }
        }
        for (java.util.Map.Entry<Character, Integer> entry : map.entrySet()) {
            if (entry.getValue() == repeatNum) {
                return entry.getKey();
            }
        }
        return '0';
    }
}