Number Of Squareful Arrays Problem
Description
LeetCode Problem 996.
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:
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Input: nums = [1,17,8]
Output: 2
Explanation: [1,8,17] and [17,8,1] are the valid permutations.
Example 2:
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Input: nums = [2,2,2]
Output: 1
Constraints:
- 1 <= nums.length <= 12
- 0 <= nums[i] <= 10^9
Sample C++ Code
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class Solution {
public:
unordered_map<int, int> count;
unordered_map<int, unordered_set<int>> cand;
int res = 0;
int numSquarefulPerms(vector<int>& A) {
for (int &a : A)
count[a]++;
for (auto &i : count) {
for (auto &j : count) {
int x = i.first, y = j.first, s = sqrt(x + y);
if (s * s == x + y)
cand[x].insert(y);
}
}
for (auto e : count)
dfs(e.first, A.size() - 1);
return res;
}
void dfs(int x, int left) {
count[x]--;
if (!left)
res++;
for (int y : cand[x])
if (count[y] > 0)
dfs(y, left - 1);
count[x]++;
}
};