LeetCode

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.

思路：

保存两个值max和leftSum，max就是到目前为止所能得到的最大值，leftSum为包含当前元素的最大子串之和。然后从左往右扫，下面的代码的if else语句把各种情况都摆明了。

package array;

public class MaximumSubarray {

    public int maxSubArray(int[] nums) {
        int n;
        if (nums == null || (n = nums.length) == 0) return 0;
        int max = nums[0];
        int leftSum = nums[0];
        for (int i = 1; i < n; ++i) {
            if (nums[i] >= 0) {
                if (leftSum >= 0) {
                    leftSum += nums[i];                    
                } else {
                    leftSum = nums[i];
                }
                
                if (max < leftSum)
                    max = leftSum;
            } else {
                if (leftSum + nums[i] >= 0) {
                    leftSum += nums[i];
                } else {
                    leftSum = nums[i];
                }
                
                if (max < leftSum)
                    max = leftSum;
            }
        }
        
        return max;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] nums = { /*-2,1,-3,4,-1,2,1,-5,4*/ -2, -1 };
        MaximumSubarray m = new MaximumSubarray();
        System.out.println(m.maxSubArray(nums));
    }

}

这个代码可以合并，如下：

package array;

public class MaximumSubarray {

    public int maxSubArray(int[] nums) {
        int n;
        if (nums == null || (n = nums.length) == 0) return 0;
        int max = nums[0];
        int leftSum = nums[0];
        for (int i = 1; i < n; ++i) {
            if (leftSum >= 0 && leftSum + nums[i] >= 0) {
                leftSum += nums[i];
            } else {
                leftSum = nums[i];
            }
            
            if (max < leftSum)
                max = leftSum;
        }
        
        return max;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] nums = { -2, -1 };
        MaximumSubarray m = new MaximumSubarray();
        System.out.println(m.maxSubArray(nums));
    }

}