LeetCode - Maximum Subarray

Find the contiguous subarray within an array (containing at least one number) which has the largest sum.

For example, given the array [−2,1,−3,4,−1,2,1,−5,4],
the contiguous subarray [4,−1,2,1] has the largest sum = 6.

More practice:

If you have figured out the O(n) solution, try coding another solution using the divide and conquer approach, which is more subtle.

Solution:

 1 public class Solution {

 2     public int maxSum(int[] a, int left, int right){

 3         if(left == right) return a[left];

 4         int mid = (left + right) / 2;

 5         int leftSum = maxSum(a, left, mid);

 6         int rightSum = maxSum(a, mid+1, right);

 7         

 8         int tmpLeftSum = 0;

 9         int tmpRightSum = 0;

10         int s1 = Integer.MIN_VALUE, s2 = Integer.MIN_VALUE;

11         for(int i = mid; i >= left; i--){

12             tmpLeftSum += a[i];

13             s1 = Math.max(s1, tmpLeftSum); 

14         }

15         for(int i = mid+1; i <= right; i++){

16             tmpRightSum += a[i];

17             s2 = Math.max(s2, tmpRightSum);

18         }

19         

20         return Math.max(Math.max(leftSum, rightSum), s1+s2);

21     }

22     

23     public int maxSum2(int[] A){

24         int len = A.length;

25         int ans = A[0];

26         int tmpSum = A[0];

27         for(int i = 1; i < len; i++){

28             if(tmpSum > 0){

29                 tmpSum = tmpSum + A[i];

30             }

31             else tmpSum = A[i];

32             ans = Math.max(ans, tmpSum);

33         }

34         return ans;

35     }

36     

37     public int maxSubArray(int[] A) {

38         // Start typing your Java solution below

39         // DO NOT write main() function

40         return maxSum(A, 0, A.length-1);

41         //return maxSum2(A);

42     }

43 }