How hard is LeetCode 679. 24 Game?

LeetCode 679. 24 Game is rated Hard on LeetCode.

What topics does LeetCode 679. 24 Game cover?

LeetCode 679. 24 Game is tagged Array, Math and Backtracking on LeetCode.

24 Game — LeetCode 679 Python Solution

HardArrayMathBacktracking

Problem: #679
Pattern: Backtracking
Reading time: 5 min
Source: leetcode.com

The problem

You are given an integer array cards of length 4. You have four cards, each containing a number in the range [1, 9].

Example

Input: cards = [4,1,8,7]
Output: true
Explanation: (8-4) * (7-1) = 24

Python solution

Python

class Solution:
    def judgePoint24(self, cards: List[int]) -> bool:
        def dfs(nums: List[float]):
            n = len(nums)
            if n == 1:
                if abs(nums[0] - 24) < 1e-6:
                    return True
                return False
            ok = False
            for i in range(n):
                for j in range(n):
                    if i != j:
                        nxt = [nums[k] for k in range(n) if k != i and k != j]
                        for op in ops:
                            match op:
                                case "/":
                                    if nums[j] == 0:
                                        continue
                                    ok |= dfs(nxt + [nums[i] / nums[j]])
                                case "*":
                                    ok |= dfs(nxt + [nums[i] * nums[j]])
                                case "+":
                                    ok |= dfs(nxt + [nums[i] + nums[j]])
                                case "-":
                                    ok |= dfs(nxt + [nums[i] - nums[j]])
                            if ok:
                                return True
            return ok

        ops = ("+", "-", "*", "/")
        nums = [float(x) for x in cards]
        return dfs(nums)

Complexity

Measure	Complexity
Time	Exponential (worst case)
Space	O(depth) auxiliary

Pattern: Backtracking

Build candidates one choice at a time and abandon a branch the moment it cannot work. LeetCode 679. 24 Game is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Backtracking.

The backtracking guide has the Python template for the pattern and the 105 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 679. 24 Game?: LeetCode 679. 24 Game is rated Hard on LeetCode.
What topics does LeetCode 679. 24 Game cover?: LeetCode 679. 24 Game is tagged Array, Math and Backtracking on LeetCode.