Palindrome Pairs

Given a list of unique words. Find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e.words[i] + words[j] is a palindrome.

Example 1:
Given words = ["bat", "tab", "cat"]
Return [[0, 1], [1, 0]]
The palindromes are ["battab", "tabbat"]

Example 2:
Given words = ["abcd", "dcba", "lls", "s", "sssll"]
Return [[0, 1], [1, 0], [3, 2], [2, 4]]
The palindromes are ["dcbaabcd", "abcddcba", "slls", "llssssll"]

class Solution {
private:
    bool isPalindrome(string s) {
        int left = 0;
        int right = s.length() - 1;

        while (left < right) {
            if (s[left++] != s[right--])
                return false;
        }
        return true;
    }
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {
        vector<vector<int>> ret;
        unordered_map<string, int> dict;
        int size = words.size();
        for (int i = 0; i < size; i++) {
            string key = words[i];
            reverse(key.begin(), key.end());
            dict[key] = i;
        }

        if (dict.find("") != dict.end()) {
            for (int i = 0; i < size; i++) {
                if (i == dict[""])
                    continue;
                if (isPalindrome(words[i]))
                    ret.push_back( { dict[""], i });
            }
        }

        for (int i = 0; i < size; i++) {
            for (int j = 0; j < words[i].size(); j++) {
                string left = words[i].substr(0, j);
                string right = words[i].substr(j, words[i].size() - j);

                if (dict.find(left) != dict.end() && isPalindrome(right)
                        && dict[left] != i) {
                    ret.push_back( { i, dict[left] });
                }
                if (dict.find(right) != dict.end() && isPalindrome(left)
                        && dict[right] != i) {
                    ret.push_back( { dict[right], i });
                }
            }
        }
        return ret;
    }
};