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 - 1 is not allowed.
一刷
backtracking
class Solution {
boolean res = false;
final double eps = 0.001;
public boolean judgePoint24(int[] nums) {
List<Double> arr = new ArrayList<>();
for(int n: nums) arr.add((double) n);
helper(arr);
return res;
}
private void helper(List<Double> arr){
if(res) return;
if(arr.size() == 1){
if(Math.abs(arr.get(0) - 24.0)<eps){
res = true;
return;
}
}
for(int i=0; i<arr.size(); i++){
for(int j=0; j<i; j++){
List<Double> next = new ArrayList<>();
Double p1 = arr.get(i), p2 = arr.get(j);
next.addAll(Arrays.asList(p1+p2, p1-p2, p2-p1, p1*p2));
if(Math.abs(p2)>eps) next.add(p1/p2);
if(Math.abs(p1)>eps) next.add(p2/p1);
arr.remove(i);
arr.remove(j);
for(Double n:next){
arr.add(n);
helper(arr);
arr.remove(arr.size()-1);
}
arr.add(j, p2);
arr.add(i, p1);
}
}
}
}
- You cannot concatenate numbers together. For example, if the input is [1, 2, 1, 2], we cannot write this as 12 + 12.
