题目:
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.
2/11/2017, Java
犯的错误:
1. 没有考虑到全是负数的情况,最简单的方法就是把每个值分为两个范围来考虑:负,非负。负值的话与当前sum比较,决定是否重置sum;非负值的话,如果当前sum为负,重置sum
2. max的初始化也可以是nums[0],min_value更直观,但是都必须保证是对的。
没有做divide and conquer approach
1 public class Solution { 2 public int maxSubArray(int[] nums) { 3 if (nums == null) return 0; 4 int max = Integer.MIN_VALUE; 5 int sum = 0; 6 7 for (int i = 0; i < nums.length; i++) { 8 if (nums[i] >= 0) { 9 if (sum < 0) sum = nums[i]; 10 else sum += nums[i]; 11 if (max < sum) max = sum; 12 } else { 13 if (sum < nums[i]) sum = nums[i]; 14 else sum += nums[i]; 15 if (max < sum) max = sum; 16 } 17 } 18 return max; 19 } 20 }
写法不够好,别人的代码。注意第8,第10行,curMax总是要加上当前值,不管正负,所以下一句第9行可以继续判断。第10行用来排除负的curMax的干扰。
1 public class Solution { 2 public int maxSubArray(int[] nums) { 3 if(nums == null || nums.length == 0) 4 return 0; 5 int globalMax = Integer.MIN_VALUE, curMax = 0; 6 7 for(int i = 0; i < nums.length; i++){ 8 curMax += nums[i]; 9 globalMax = Math.max(globalMax, curMax); 10 if(curMax < 0) 11 curMax = 0; 12 } 13 14 return globalMax; 15 } 16 }