leetcood学习笔记-53*-最大子列和

题目描述：

方法一：O(N)

class Solution(object):
    def maxSubArray(self, nums):
        sum = 0
        max_sub_sum = nums[0]
        for num in nums:
            sum += num
            if sum > max_sub_sum:
                max_sub_sum = sum
            if sum < 0:
                sum = 0
        return max_sub_sum

二：O(N)/O(1)

class Solution(object):
    def maxSubArray(self, nums):
        """
        :type nums: List[int]
        :rtype: int
         """
        for i in range(1, len(nums)):
            nums[i]= nums[i] + max(nums[i-1], 0)
        return max(nums)

三：分治法 O(logn) O(logn)

class Solution {
    public int maxSubArray(int[] nums) {
        if (nums == null || nums.length <= 0)
            return 0;
        int len = nums.length;
        return getInfo(nums,0,len - 1).mSum;
    }
    class wtevTree{
        int lSum;
        int rSum;
        int iSum;
        int mSum;

        wtevTree(int l, int r, int i, int m){
            lSum = l;
            rSum = r;
            iSum = i;
            mSum = m;
        }
    }
    wtevTree pushUp(wtevTree leftT,wtevTree rightT){
        int l = Math.max(leftT.lSum,leftT.iSum + rightT.lSum);
        int r = Math.max(leftT.rSum + rightT.iSum,rightT.rSum);
        int i = leftT.iSum + rightT.iSum;
        int m = Math.max(leftT.rSum + rightT.lSum,Math.max(leftT.mSum,rightT.mSum));
        return new wtevTree(l,r,i,m);
    }

    wtevTree getInfo(int[] nums,int left,int right){
        if(left == right)
            return new wtevTree(nums[left],nums[left],nums[left],nums[left]);
        int mid = (left + right) >> 1;
        wtevTree leftT = getInfo(nums,left,mid);
        wtevTree rightT = getInfo(nums,mid + 1,right);
        return pushUp(leftT,rightT);
    }
}