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:
Input: [4, 1, 8, 7]
Output: True
Explanation: (8-4) * (7-1) = 24
Example 2:
Input: [1, 2, 1, 2]
Output: False
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 - 1is not allowed. - You cannot concatenate numbers together. For example, if the input is
[1, 2, 1, 2], we cannot write this as 12 + 12.
思路：把每两个数拿出来，找出其所有的加减乘除组合，然后把每一个取出来，后剩余的两个数（总共也就是3个数），继续做同样的操作，直到只剩下一个数，看那个数是不是24.
class Solution {
public boolean judgePoint24(int[] nums) {
return helper(new double[] { nums[], nums[], nums[], nums[] });
}
private boolean helper(double[] nums) {
if (nums.length == ) return Math.abs(nums[] - ) < 0.01;
for (int i = ; i < nums.length; i++) {
for (int j = i + ; j < nums.length; j++) {
double[] remainings = new double[nums.length - ];
int idx = ;
// add numbers in nums to remainings excluding nums[i] and nums[j]
for (int k = ; k < nums.length; k++) {
if (k != i && k != j) {
remainings[idx] = nums[k];
idx++;
}
}
for (double k : computeCombinations(nums[i], nums[j])) {
remainings[remainings.length - ] = k;
if (helper(remainings)) {
return true;
}
}
}
}
return false;
}
private double[] computeCombinations(double a, double b) {
return new double[] { a + b, a - b, b - a, a * b, a / b, b / a };
}
}