24 Game
Hard · Backtracking · about 30 minutes
Asked at Google, Uber, Microsoft, ByteDance, Bloomberg.
You have 4 cards, each a number from 1 to 9. Using +, -, *, / (real division) and parentheses, can you make an expression equal to 24 that uses every card exactly once? Unary minus and concatenating digits are not allowed. Return true or false.