679 24 Game
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You have 4 cards each containing a number from 1 to 9. You need to judge whether they could operated through*
,/
,+
,-
,(
,)
to get the value of 24.
Example 1:
Example 2:
Note:
The division operator/
represents real division, not integer division. For example, 4 / (1 - 2/3) = 12.
Every operation done is between two numbers. In particular, we cannot use-
as a unary operator. For example, with[1, 1, 1, 1]
as input, the expression -1 - 1 - 1 - 1
is not allowed.
You cannot concatenate numbers together. For example, if the input is[1, 2, 1, 2]
, we cannot write this as 12 + 12.
(1) Backtracking
Backtracking: 时间复杂度O(1) ? 空间复杂度O(1)?