题目：

题解：

// 哈希表实现
class Solution {
    public List<List<Integer>> palindromePairs(String[] words) {
        List<List<Integer>> res = new ArrayList<>();
        int n = words.length;
        Map<String, Integer> indices = new HashMap<>();
        for (int i = 0; i < n; ++i) {
            String w = new StringBuilder(words[i]).reverse().toString();
            indices.put(w, i);
        }
        for (int i = 0; i < n; ++i) {
            int m = words[i].length();
            if (m == 0) {
                continue;
            }
            for (int j = 0; j <= m; ++j) {
                if (isPalindrome(words[i], j, m - 1)) {
                    String w = words[i].substring(0, j);
                    int leftId = indices.getOrDefault(w, -1);
                    if (leftId != -1 && leftId != i) {
                        res.add(Arrays.asList(i, leftId));
                    }
                }
                if (j != 0 && isPalindrome(words[i], 0, j - 1)) {
                    String w = words[i].substring(j, m);
                    int rightId = indices.getOrDefault(w, -1);
                    if (rightId != -1 && rightId != i) {
                        res.add(Arrays.asList(rightId, i));
                    }
                }
            }
        }
        return res;
    }

    public boolean isPalindrome(String s, int left, int right) {
        int n = right - left + 1;
        for (int i = 0; i < n / 2; ++i) {
            if (s.charAt(left + i) != s.charAt(right - i)) {
                return false;
            }
        }
        return true;
    }
}