力扣算法题—053最大子序之和

#include "000库函数.h"

//一想到又是遍历
class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        int res=nums[0];
        for (int i = 0; i < nums.size(); ++i) {
            int sum = nums[i];
            res = res > sum ? res : sum;
            for (int j = i + 1; j < nums.size(); ++j) {
                sum += nums[j];
                res = res > sum ? res : sum;
            }
        }
        return res;
    }
};

//使用滑窗来做
class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        int res = nums[0];
        int sum = -1;
        for (auto a : nums) {
            sum = max(a, sum + a);
            res = max(res, sum);
        }
        return res;
    }
};



//用分治法来计算，将数组一分为二，从中间来开始遍历

class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        return helper(nums, 0, nums.size() - 1);
    }
    int helper(vector<int>nums, int left, int right) {
        if (left >= right)return nums[left];
        int mid = (left + right) / 2;
        int lmax = helper(nums, left, mid - 1);//左移
        int rmax = helper(nums, mid + 1, right);//右移
        int mmax = nums[mid], t = mmax;
        for (int i = mid - 1; i >= left; --i) {//左移计算
            t += nums[i];
            mmax = mmax > t ? mmax : t;
        }
        t = mmax;
        for (int i = mid + 1; i <= right; ++i) {
            t += nums[i];
            mmax = mmax > t ? mmax : t;
        }
        return mmax > (rmax > lmax ? rmax : lmax) ? mmax : (rmax > lmax ? rmax : lmax);
    }
};

void T053() {
    Solution s;
    vector<int>v;
    v = { -2, 1, -3, 4, -1, 2, 1, -5, 4 };
    cout << s.maxSubArray(v) << endl;
}