https://leetcode-cn.com/problems/maximum-subarray/
状态表示: f[i]表示以i结尾的最大子段和

即f[i]=max(f[i-1]+nums[i],nums[i]) => f[i]=max(f[i-1],0)+nums[i]

class Solution {
public:
    int f[100100];
    int maxSubArray(vector<int>& nums) 
    {
       for(int i=0;i<nums.size();i++) f[i]=-1e9;
       f[0]=nums[0];
       for(int i=0;i<nums.size();i++) if(i) f[i]=max(f[i-1],0)+nums[i];
       int sum=nums[0];
       for(int i=0;i<nums.size();i++) sum=max(sum,f[i]);
       return sum;
    }
};

class Solution {
public:
    int maxSubArray(vector<int>& nums) 
    {
       int last=-1e9,ans=-1e9;
       for(int i=0;i<nums.size();i++) 
       {
           last=max(last,0)+nums[i];
           ans=max(ans,last);
       }
       return ans;
    }
};