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]
andtarget
consist of digits. -
nums[i]
andtarget
do not have leading zeros.
SOLUTION:
from collections import Counter
class Solution:
def numOfPairs(self, nums: List[str], target: str) -> int:
k = len(target)
nums = Counter(nums)
ctr = 0
for num in nums:
if target.startswith(num):
chunk = target[len(num) - k:]
if chunk in nums:
ctr += nums[num] * nums[chunk]
if num == chunk:
ctr -= nums[chunk]
return ctr
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