996. Number of Squareful Arrays
Đề Bài
An array is squareful if the sum of every pair of adjacent elements is a perfect square.
Given an integer array nums, return the number of permutations of nums that are squareful.
Two permutations perm1 and perm2 are different if there is some index i such that perm1[i] != perm2[i].
Example 1:
Input: nums = [1,17,8] Output: 2 Explanation: [1,8,17] and [17,8,1] are the valid permutations.
Example 2:
Input: nums = [2,2,2] Output: 1
Constraints:
1 <= nums.length <= 120 <= nums[i] <= 109
Thuật Toán & Kỹ Thuật
⏱️ Thời gian
O(V+E)
💾 Không gian
O(V)
Lời Giải
C++
0996-number-of-squareful-arrays.cpp
class Solution {
private:
vector<int> nums, path;
unordered_map<int, int> cnt;
int n, ans;
public:
bool checkSquare(int num) {
int s = sqrt(num);
return num == s*s;
}
void dfs(int prev) {
if (path.size() == n) {
ans++;
return;
}
for (auto &[key, _] : cnt) {
if (cnt[key] > 0) {
if (prev == -1 or checkSquare(prev + key)) {
cnt[key]--;
path.push_back(key);
dfs(key);
cnt[key]++;
path.pop_back();
}
}
}
}
int numSquarefulPerms(vector<int>& nums) {
n = nums.size(), ans = 0;
this->nums = nums;
for (int num : nums) {
cnt[num]++;
}
dfs(-1);
return ans;
}
};