Given a set of unique words, find the pairs of indices (i, j) that are distinct in the given list such that two words associated, eg: words[i] + words[j] form a palindrome.
Example 1:
Given words = ["bat", "tab", "cat"]
return [[0, 1], [1, 0]]
palindrome is ["battab", "tabbat"]
Example 2:
give Given words = ["abcd", "dcba", "lls", "s", "sssll"]
returns [[0, 1], [1, 0], [3, 2], [2, 4]]
The palindrome is ["dcbaabcd", "abcddcba", "slls", "llssssll"]
See: https://leetcode.com/problems/palindrome-pairs/description/
C++:
class Solution { public: vector<vector<int>> palindromePairs(vector<string>& words) { vector<vector<int>> res; unordered_map<string, int> m; set<int> s; for (int i = 0; i < words.size(); ++i) { m[words[i]] = i; s.insert(words[i].size()); } for (int i = 0; i < words.size(); ++i) { string t = words[i]; int len = t.size(); reverse(t.begin(), t.end()); if (m.count(t) && m[t] != i) { res.push_back({i, m[t]}); } auto a = s.find (len); for (auto it = s.begin(); it != a; ++it) { int d = *it; if (isValid(t, 0, len - d - 1) && m.count(t.substr(len - d))) { res.push_back({i, m[t.substr(len - d)]}); } if (isValid(t, d, len - 1) && m.count(t.substr(0, d))) { res.push_back({m[t.substr(0, d)], i}); } } } return res; } bool isValid(string t, int left, int right) { while (left < right) { if (t[left++] != t[right--]) { return false; } } return true; } };
Reference: https://leetcode.com/problems/palindrome-pairs/description/