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718. Maximum Length of Repeated Subarray

Description

Given two integer arrays nums1 and nums2, return the maximum length of a subarray that appears in both arrays.

 

Example 1:

Input: nums1 = [1,2,3,2,1], nums2 = [3,2,1,4,7]
Output: 3
Explanation: The repeated subarray with maximum length is [3,2,1].

Example 2:

Input: nums1 = [0,0,0,0,0], nums2 = [0,0,0,0,0]
Output: 5
Explanation: The repeated subarray with maximum length is [0,0,0,0,0].

 

Constraints:

  • 1 <= nums1.length, nums2.length <= 1000
  • 0 <= nums1[i], nums2[i] <= 100

Solutions

Solution 1

Thinking

Find the longest common contiguous subarray. Lengths are \(1000\), so matching from every pair of starts is too slow, and LCS DP does not enforce contiguity.

Contiguity means a suffix grows only when the current pair matches: it is one plus the suffix of the prefixes, otherwise zero.

Let \(f[i][j]\) be the common suffix ending at \(nums1[i-1]\) and \(nums2[j-1]\). Take the global maximum. Time \(O(mn)\).

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class Solution:
    def findLength(self, nums1: List[int], nums2: List[int]) -> int:
        m, n = len(nums1), len(nums2)
        f = [[0] * (n + 1) for _ in range(m + 1)]
        ans = 0
        for i in range(1, m + 1):
            for j in range(1, n + 1):
                if nums1[i - 1] == nums2[j - 1]:
                    f[i][j] = f[i - 1][j - 1] + 1
                    ans = max(ans, f[i][j])
        return ans

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class Solution {
    public int findLength(int[] nums1, int[] nums2) {
        int m = nums1.length;
        int n = nums2.length;
        int[][] f = new int[m + 1][n + 1];
        int ans = 0;
        for (int i = 1; i <= m; ++i) {
            for (int j = 1; j <= n; ++j) {
                if (nums1[i - 1] == nums2[j - 1]) {
                    f[i][j] = f[i - 1][j - 1] + 1;
                    ans = Math.max(ans, f[i][j]);
                }
            }
        }
        return ans;
    }
}

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class Solution {
public:
    int findLength(vector<int>& nums1, vector<int>& nums2) {
        int m = nums1.size(), n = nums2.size();
        vector<vector<int>> f(m + 1, vector<int>(n + 1));
        int ans = 0;
        for (int i = 1; i <= m; ++i) {
            for (int j = 1; j <= n; ++j) {
                if (nums1[i - 1] == nums2[j - 1]) {
                    f[i][j] = f[i - 1][j - 1] + 1;
                    ans = max(ans, f[i][j]);
                }
            }
        }
        return ans;
    }
};

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func findLength(nums1 []int, nums2 []int) (ans int) {
    m, n := len(nums1), len(nums2)
    f := make([][]int, m+1)
    for i := range f {
        f[i] = make([]int, n+1)
    }
    for i := 1; i <= m; i++ {
        for j := 1; j <= n; j++ {
            if nums1[i-1] == nums2[j-1] {
                f[i][j] = f[i-1][j-1] + 1
                if ans < f[i][j] {
                    ans = f[i][j]
                }
            }
        }
    }
    return ans
}

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function findLength(nums1: number[], nums2: number[]): number {
    const m = nums1.length;
    const n = nums2.length;
    const f = Array.from({ length: m + 1 }, _ => new Array(n + 1).fill(0));
    let ans = 0;
    for (let i = 1; i <= m; ++i) {
        for (let j = 1; j <= n; ++j) {
            if (nums1[i - 1] == nums2[j - 1]) {
                f[i][j] = f[i - 1][j - 1] + 1;
                ans = Math.max(ans, f[i][j]);
            }
        }
    }
    return ans;
}

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/**
 * @param {number[]} nums1
 * @param {number[]} nums2
 * @return {number}
 */
var findLength = function (nums1, nums2) {
    const m = nums1.length;
    const n = nums2.length;
    const f = Array.from({ length: m + 1 }, _ => new Array(n + 1).fill(0));
    let ans = 0;
    for (let i = 1; i <= m; ++i) {
        for (let j = 1; j <= n; ++j) {
            if (nums1[i - 1] == nums2[j - 1]) {
                f[i][j] = f[i - 1][j - 1] + 1;
                ans = Math.max(ans, f[i][j]);
            }
        }
    }
    return ans;
};

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