README_EN.md

1191. K-Concatenation Maximum Sum

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Description

Given an integer array arr and an integer k, modify the array by repeating it k times.

For example, if arr = [1, 2] and k = 3 then the modified array will be [1, 2, 1, 2, 1, 2].

Return the maximum sub-array sum in the modified array. Note that the length of the sub-array can be 0 and its sum in that case is 0.

As the answer can be very large, return the answer modulo 109 + 7.

 

Example 1:

Input: arr = [1,2], k = 3
Output: 9

Example 2:

Input: arr = [1,-2,1], k = 5
Output: 2

Example 3:

Input: arr = [-1,-2], k = 7
Output: 0

 

Constraints:

• 1 <= arr.length <= 105
• 1 <= k <= 105
• -104 <= arr[i] <= 104

Solutions

Solution 1: Prefix Sum + Case Discussion

We denote the sum of all elements in the array $arr$ as $s$, the maximum prefix sum as $mxPre$, the minimum prefix sum as $miPre$, and the maximum subarray sum as $mxSub$.

We traverse the array $arr$. For each element $x$, we update $s=s+x$, $mxPre=max(mxPre,s)$, $miPre=min(miPre,s)$, $mxSub=max(mxSub,s-miPre)$.

Next, we consider the value of $k$:

• When $k=1$, the answer is $mxSub$.
• When $k\ge 2$, if the maximum subarray spans two $arr$, then the answer is $mxPre+mxSuf$, where $mxSuf=s-miPre$.
• When $k\ge 2$ and $s>0$, if the maximum subarray spans three $arr$, then the answer is $(k-2)\times s+mxPre+mxSuf$.

Finally, we return the result of the answer modulo ${10}^9+7$.

The time complexity is $O(n)$, and the space complexity is $O(1)$. Here, $n$ is the length of the array $arr$.

class Solution:
    def kConcatenationMaxSum(self, arr: List[int], k: int) -> int:
        s = mx_pre = mi_pre = mx_sub = 0
        for x in arr:
            s += x
            mx_pre = max(mx_pre, s)
            mi_pre = min(mi_pre, s)
            mx_sub = max(mx_sub, s - mi_pre)
        ans = mx_sub
        mod = 10**9 + 7
        if k == 1:
            return ans % mod
        mx_suf = s - mi_pre
        ans = max(ans, mx_pre + mx_suf)
        if s > 0:
            ans = max(ans, (k - 2) * s + mx_pre + mx_suf)
        return ans % mod

class Solution {
    public int kConcatenationMaxSum(int[] arr, int k) {
        long s = 0, mxPre = 0, miPre = 0, mxSub = 0;
        for (int x : arr) {
            s += x;
            mxPre = Math.max(mxPre, s);
            miPre = Math.min(miPre, s);
            mxSub = Math.max(mxSub, s - miPre);
        }
        long ans = mxSub;
        final int mod = (int) 1e9 + 7;
        if (k == 1) {
            return (int) (ans % mod);
        }
        long mxSuf = s - miPre;
        ans = Math.max(ans, mxPre + mxSuf);
        if (s > 0) {
            ans = Math.max(ans, (k - 2) * s + mxPre + mxSuf);
        }
        return (int) (ans % mod);
    }
}

class Solution {
public:
    int kConcatenationMaxSum(vector<int>& arr, int k) {
        long s = 0, mxPre = 0, miPre = 0, mxSub = 0;
        for (int x : arr) {
            s += x;
            mxPre = max(mxPre, s);
            miPre = min(miPre, s);
            mxSub = max(mxSub, s - miPre);
        }
        long ans = mxSub;
        const int mod = 1e9 + 7;
        if (k == 1) {
            return ans % mod;
        }
        long mxSuf = s - miPre;
        ans = max(ans, mxPre + mxSuf);
        if (s > 0) {
            ans = max(ans, mxPre + (k - 2) * s + mxSuf);
        }
        return ans % mod;
    }
};

func kConcatenationMaxSum(arr []int, k int) int {
	var s, mxPre, miPre, mxSub int
	for _, x := range arr {
		s += x
		mxPre = max(mxPre, s)
		miPre = min(miPre, s)
		mxSub = max(mxSub, s-miPre)
	}
	const mod = 1e9 + 7
	ans := mxSub
	if k == 1 {
		return ans % mod
	}
	mxSuf := s - miPre
	ans = max(ans, mxSuf+mxPre)
	if s > 0 {
		ans = max(ans, mxSuf+(k-2)*s+mxPre)
	}
	return ans % mod
}