LeetCode 1794. Count Pairs of Equal Substrings With Minimum Difference

You are given two strings firstString and secondString that are 0-indexed and consist only of lowercase English letters. Count the number of index quadruples (i,j,a,b) that satisfy the following conditions:

• 0 <= i <= j < firstString.length

• 0 <= a <= b < secondString.length

• The substring of firstString that starts at the ith character and ends at the jthcharacter (inclusive) is equal to the substring of secondString that starts at the athcharacter and ends at the bth character (inclusive).

• j - a is the minimum possible value among all quadruples that satisfy the previous conditions.

Return the number of such quadruples.

Example 1:

Input: firstString = "abcd", secondString = "bccda"
Output: 1
Explanation: The quadruple (0,0,4,4) is the only one that satisfies all the conditions and minimizes j - a.

Example 2:

Input: firstString = "ab", secondString = "cd"
Output: 0
Explanation: There are no quadruples satisfying all the conditions.

Constraints:

• 1 <= firstString.length, secondString.length <= 2 * 10^5

• Both strings consist only of lowercase English letters.

Solution

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中文版解答Bilibili Link

class Solution {
public:
    int countQuadruples(string firstString, string secondString) {
        unordered_map<char, int> last_occurence;
        for (int i = 0; i < secondString.length(); i++) {
            last_occurence[secondString[i]] = i;
        }
        
        int minimum_j_minus_a = INT_MAX;
        int cnt = 0;
        for (int i = 0; i < firstString.length(); i++) {
            char ch = firstString[i];
            if (last_occurence.count(ch) > 0) {
                int j_minus_a = i - last_occurence[ch];
                if (j_minus_a < minimum_j_minus_a) {
                    minimum_j_minus_a = j_minus_a;
                    cnt = 1;
                } else if (j_minus_a == minimum_j_minus_a) {
                    cnt++;
                }
            }
        }
        
        return cnt;
    }
};