Leetcode #2023: Number of Pairs of Strings With Concatenation Equal to Target
In this guide, we solve Leetcode #2023 Number of Pairs of Strings With Concatenation Equal to Target in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given an array of digit strings nums and a digit string target, return the number of pairs of indices (i, j) (where i != j) such that the concatenation of nums[i] + nums[j] equals target. Example 1: Input: nums = ["777","7","77","77"], target = "7777" Output: 4 Explanation: Valid pairs are: - (0, 1): "777" + "7" - (1, 0): "7" + "777" - (2, 3): "77" + "77" - (3, 2): "77" + "77" Example 2: Input: nums = ["123","4","12","34"], target = "1234" Output: 2 Explanation: Valid pairs are: - (0, 1): "123" + "4" - (2, 3): "12" + "34" Example 3: Input: nums = ["1","1","1"], target = "11" Output: 6 Explanation: Valid pairs are: - (0, 1): "1" + "1" - (1, 0): "1" + "1" - (0, 2): "1" + "1" - (2, 0): "1" + "1" - (1, 2): "1" + "1" - (2, 1): "1" + "1" Constraints: 2 <= nums.length <= 100 1 <= nums[i].length <= 100 2 <= target.length <= 100 nums[i] and target consist of digits.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array, Hash Table, String, Counting
Intuition
Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.
By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.
Approach
Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.
This keeps the solution linear while remaining easy to explain in an interview setting.
Steps:
- Initialize a hash map for seen items or counts.
- Iterate through the input, querying/updating the map.
- Return the first valid result or the final computed value.
Example
Input: nums = ["777","7","77","77"], target = "7777"
Output: 4
Explanation: Valid pairs are:
- (0, 1): "777" + "7"
- (1, 0): "7" + "777"
- (2, 3): "77" + "77"
- (3, 2): "77" + "77"
Python Solution
class Solution:
def numOfPairs(self, nums: List[str], target: str) -> int:
n = len(nums)
return sum(
i != j and nums[i] + nums[j] == target for i in range(n) for j in range(n)
)
Complexity
The time complexity is , where and are the lengths of the array nums and the string target, respectively. The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.