LintCode-Maximum Subarray III

Given an array of integers and a number k, find k non-overlapping subarrays which have the largest sum.

The number in each subarray should be contiguous.

Return the largest sum.

Note

The subarray should contain at least one number

Analysis:

DP. d[i][j] means the maximum sum we can get by selecting j subarrays from the first i elements.

d[i][j] = max{d[p][j-1]+maxSubArray(p+1,i)}

we iterate p from i-1 to j-1, so we can record the max subarray we get at current p, this value can be used to calculate the max subarray from p-1 to i when p becomes p-1.

Solution:

 1 public class Solution {
 2     /**
 3      * @param nums: A list of integers
 4      * @param k: An integer denote to find k non-overlapping subarrays
 5      * @return: An integer denote the sum of max k non-overlapping subarrays
 6      */
 7     public int maxSubArray(ArrayList<Integer> nums, int k) {
 8         if (nums.size()<k) return 0;
 9         int len = nums.size();
10         //d[i][j]: select j subarrays from the first i elements, the max sum we can get.
11         int[][] d = new int[len+1][k+1];
12         for (int i=0;i<=len;i++) d[i][0] = 0;        
13         
14         for (int j=1;j<=k;j++)
15             for (int i=j;i<=len;i++){
16                 d[i][j] = Integer.MIN_VALUE;
17                 //Initial value of endMax and max should be taken care very very carefully.
18                 int endMax = 0;
19                 int max = Integer.MIN_VALUE;                
20                 for (int p=i-1;p>=j-1;p--){
21                     endMax = Math.max(nums.get(p), endMax+nums.get(p));
22                     max = Math.max(endMax,max);
23                     if (d[i][j]<d[p][j-1]+max)
24                         d[i][j] = d[p][j-1]+max;                    
25                 }
26             }
27 
28         return d[len][k];
29                     
30 
31     }
32 }

Solution 2:

Use one dimension array.

 1 public class Solution {
 2     /**
 3      * @param nums: A list of integers
 4      * @param k: An integer denote to find k non-overlapping subarrays
 5      * @return: An integer denote the sum of max k non-overlapping subarrays
 6      */
 7     public int maxSubArray(ArrayList<Integer> nums, int k) {
 8         if (nums.size()<k) return 0;
 9         int len = nums.size();
10         //d[i][j]: select j subarrays from the first i elements, the max sum we can get.
11         int[] d = new int[len+1];
12         for (int i=0;i<=len;i++) d[i] = 0;        
13         
14         for (int j=1;j<=k;j++)
15             for (int i=len;i>=j;i--){
16                 d[i] = Integer.MIN_VALUE;
17                 int endMax = 0;
18                 int max = Integer.MIN_VALUE;                
19                 for (int p=i-1;p>=j-1;p--){
20                     endMax = Math.max(nums.get(p), endMax+nums.get(p));
21                     max = Math.max(endMax,max);
22                     if (d[i]<d[p]+max)
23                         d[i] = d[p]+max;                    
24                 }
25             }
26 
27         return d[len];
28                     
29 
30     }
31 }

 

 

posted @ 2014-12-25 09:38  LiBlog  阅读(4346)  评论(2)  编辑  收藏  举报