01-复杂度1. 最大子列和问题(20)

 1 import java.util.Scanner;
 2 
 3 //一个文件中只能有一个共有的类，并且与文件名称一致，大小写注意
 4 public class Main{
 5   // 程序的入口
 6   public static void main(String args[]){
 7       Scanner sc = new Scanner(System.in);
 8       String num = sc.nextLine();
 9       String[] str = sc.nextLine().split(" ");
10       sc.close();
11      
12       
13       int n = Integer.parseInt(num);
14       int[] numArr = new int[n];
15       for(int i=0;i<n;i++){
16           numArr[i] = Integer.parseInt(str[i]);
17       }
18 
19       int result  = maxSumRec(numArr);
20       System.out.print(result);
21       
22   }
23 
24   private static int maxSumRec(int[] a) {
25     // TODO Auto-generated method stub
26     return maxSumRec(a,0,a.length-1);
27   }
28 
29   /**
30    * 求出数组a中的最大子序列和，如果全是负数，返回0
31    * @param a 
32    * @param left 左边界
33    * @param right 右边界
34    * @return 
35    */
36   private static int maxSumRec(int[] a, int left, int right) {
37     //base case
38     if(left == right){
39       if(a[left]>0){
40           return a[left];
41       }else{
42           return 0;
43       }    
44     }
45     
46     int center = (left + right)/2;
47     int maxLeftSum = maxSumRec(a,left,center);
48     int maxRightSum = maxSumRec(a,center+1,right);
49     
50     int maxLeftBorderSum = 0,leftBorderSum = 0;
51     for(int i=center;i>=left;i--){
52         leftBorderSum += a[i];
53         if(leftBorderSum>maxLeftBorderSum){
54             maxLeftBorderSum = leftBorderSum;
55         }
56     }
57     
58     int maxRightBorderSum = 0,rightBorderSum = 0;
59     for(int i=center+1;i<=right;i++){
60         rightBorderSum += a[i];
61         if(rightBorderSum>maxRightBorderSum){
62             maxRightBorderSum = rightBorderSum;
63         }
64     }
65     
66     return max3(maxLeftSum,maxRightSum,maxRightBorderSum+maxLeftBorderSum);
67   }
68   
69   private static int max3(int a, int b, int c) {
70       int max = 0;
71       if(a>b){
72         if(a>c){
73             max = a;
74         }else{
75             max = c;
76         }
77       }else{
78           if(b>c){
79              max = b;
80           }else{
81               max = c;
82           }
83       }
84     return max;
85   }
86   
87 }

 1 import java.util.Scanner;
 2 
 3 //一个文件中只能有一个共有的类，并且与文件名称一致，大小写注意
 4 public class Main{
 5   // 程序的入口
 6   public static void main(String args[]){
 7       Scanner sc = new Scanner(System.in);
 8       String num = sc.nextLine();
 9       String[] str = sc.nextLine().split(" ");
10       sc.close();
11      
12       
13       int n = Integer.parseInt(num);
14       int[] numArr = new int[n];
15       for(int i=0;i<n;i++){
16           numArr[i] = Integer.parseInt(str[i]);
17       }
18 
19       int result  = maxSumRec4(numArr);
20       System.out.print(result);
21       
22   }
23 
24   private static int maxSumRec4(int[] a) {
25         int maxSum = 0; 
26         int thisSum = 0;
27         for(int j=0;j<a.length;j++){
28           thisSum += a[j];
29           if(thisSum>maxSum){
30              maxSum = thisSum;
31           }else if(thisSum<0){
32             thisSum = 0;
33           }
34         }
35         
36         return maxSum;
37   }
38 }