No.278 - LeetCode[1031] Maximum Sum of Two Non-Overlapping Subarrays - 前缀和

思路如下代码，可以进一步内存优化，取消dp数组，直接用sum即可。

/*
 * @lc app=leetcode id=1031 lang=cpp
 *
 * [1031] Maximum Sum of Two Non-Overlapping Subarrays
 */

// @lc code=start
class Solution {
public:
    int maxSumTwoNoOverlap(vector<int>& nums, int firstLen, int secondLen) {
        int N = nums.size();
        int sumAL=0, sumAR=0, sumBL=0, sumBR = 0;
        vector<int> dpAL(N+2, 0), dpAR(N+2, 0);
        vector<int> dpBL(N+2, 0), dpBR(N+2, 0);
        for(int i=0;i<firstLen-1;i++){
            sumAL += nums[i];
        }
        for(int i=0;i<secondLen-1;i++){
            sumBL += nums[i];
        }
        for(int i=firstLen;i<=N;i++){
            sumAL += nums[i-1];
            dpAL[i] = max(dpAL[i-1], sumAL);
            sumAL -= nums[i-firstLen];
        }
        for(int i=secondLen;i<=N;i++){
            sumBL += nums[i-1];
            dpBL[i] = max(dpBL[i-1], sumBL);
            sumBL -= nums[i-secondLen];
        }

        for(int i=N-1;i>N-firstLen;i--){
            sumAR += nums[i];
        }
        for(int i=N-1;i>N-secondLen;i--){
            sumBR += nums[i];
        }
        for(int i=N-firstLen;i>0;i--){
            sumAR += nums[i];
            dpAR[i] = max(dpAR[i+1], sumAR);
            sumAR -= nums[i+firstLen-1];
        }
        for(int i=N-secondLen;i>0;i--){
            sumBR += nums[i];
            dpBR[i] = max(dpBR[i+1], sumBR);
            sumBR -= nums[i+secondLen-1];
        }
        int ans = 0;
        for(int i=1;i<=N;i++){
            ans = max(ans, dpAL[i] + dpBR[i]);
            ans = max(ans, dpBL[i] + dpAR[i]);
        }
        return ans;
    }
};
// @lc code=end