NC_18_ROTATEMATRIX-NC_19_maxsumofSubarray-NC_20_restoreIpAddresses

package org.example.interview.practice;

/**
 * @author xianzhe.ma
 * @date 2021/8/14
 */

public class NC_18_ROTATEMATRIX {

    public int[][] rotateMatrix(int[][] mat, int n) {
        // write code here
        // 对角线
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                int temp = mat[i][j];
                mat[i][j] = mat[j][i];
                mat[j][i] = temp;
            }
        }

        // 以竖轴翻转
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n / 2; j++) {
                int temp = mat[i][j];
                mat[i][j] = mat[i][n - 1 - j];
                mat[i][n - 1 - j] = temp;
            }
        }
        return mat;
    }
}

package org.example.interview.practice;

/**
 * @author xianzhe.ma
 * @date 2021/8/31
 */

public class NC_19_maxsumofSubarray {

    public static int maxsumofSubarray(int[] arr) {
        // write code here
        //存放临时答案
        int thisSum = 0;
        //存放最终答案，注意初始化的值
        int ans = Integer.MIN_VALUE;
        int len = arr.length;
        for (int i = 0; i < len; i++) {
            //累加求和
            thisSum += arr[i];
            if (thisSum > ans) {
                ans = thisSum;
            }
            //贪心，负数必然会拉低序列和
            if (thisSum < 0) {
                thisSum = 0;
            }
        }
        return ans;
    }

    public static int maxsumofSubarray2(int[] arr) {
        // write code here
        int sum = 0;
        int max = Integer.MIN_VALUE;

        for (int value : arr) {
            sum = sum + value;
            if (sum > max) {
                max = sum;
            }
            if (sum < 0) {
                sum = 0;
            }
        }

        return max;
    }

    public static void main(String[] args) {
        int[] arr = {1, 2, -99, 5, -5};
        System.out.println(maxsumofSubarray2(arr));
    }
}

package org.example.interview.practice;

import java.util.ArrayList;

/**
 * @author xianzhe.ma
 * @date 2021/8/31
 */

public class NC_20_restoreIpAddresses {
    /**
     *
     * @param s string字符串
     * @return string字符串ArrayList
     */
    public ArrayList<String> restoreIpAddresses (String s) {
        // write code here
        ArrayList<String> list=new ArrayList<>();
        if(s.length()==0)return list;
        backTrack(s,0,3,list);
        return list;
    }
    //i:本次插入的起始位置 cnt:剩余可插入'.'的次数
    public void backTrack(String s,int i,int cnt,ArrayList<String> list){
        if(cnt==0){
            String[] strs=s.split("\\."); //'.'要转义
            if(strs.length<4)return ; //保证插入得到4个子串，不会出现多个'.'相连的情况
            for(String str:strs){
                if(str.length()>1&&str.charAt(0)=='0')return ; //排除有前导0的情况
                if(Integer.parseInt(str)<0||Integer.parseInt(str)>255)return ; //排除不在范围的情况
            }
            list.add(s);
            return ;
        }
        if(i>=s.length())return ; //没插完就结束的情况
        int n=s.length();
        backTrack(s.substring(0,i+1)+"."+s.substring(i+1,n),i+2,cnt-1,list); //插入到1个字符之后
        if(i+2<n)backTrack(s.substring(0,i+2)+"."+s.substring(i+2,n),i+3,cnt-1,list); //插入到2个字符之后
        if(i+3<n)backTrack(s.substring(0,i+3)+"."+s.substring(i+3,n),i+4,cnt-1,list); //插入到3个字符之后
    }
}