思路：
字典树。
实现：
1 class TrieNode 2 { 3 public: 4 int v[26]; 5 int id = -1; 6 TrieNode() 7 { 8 fill(v, v + 26, -1); 9 } 10 }; 11 TrieNode pool[1500001]; 12 class Trie 13 { 14 public: 15 int root = -1; 16 int cnt = 0; 17 Trie() 18 { 19 root = cnt++; 20 } 21 inline void insert(string& s, int id) 22 { 23 int l = s.length(), cur = root; 24 for (int i = 0; i < l; i++) 25 { 26 int id = s[i]-'a'; 27 if (pool[cur].v[id] == -1) 28 { 29 pool[cur].v[id] = cnt++; 30 } 31 cur = pool[cur].v[id]; 32 } 33 pool[cur].id = id; 34 } 35 inline int search(string& s, int high, int low) // high >= low 36 { 37 if (s.empty() or high < low) 38 { 39 if (root == -1) return -1; 40 return pool[root].id; 41 } 42 int l = s.length(); 43 int cur = root; 44 for (int i = high; i >= low; i--) 45 { 46 int id = s[i]-'a'; 47 if (pool[cur].v[id] == -1) return -1; 48 cur = pool[cur].v[id]; 49 } 50 return pool[cur].id; 51 } 52 inline void _del(int root) 53 { 54 if (root < 0) return; 55 for (int i = 0; i < 26; i++) 56 { 57 _del(pool[root].v[i]); 58 pool[root].v[i] = -1; 59 } 60 pool[root].id = -1; 61 } 62 inline void del() 63 { 64 _del(root); 65 } 66 }; 67 class Solution 68 { 69 public: 70 inline bool check(string& s, int l, int r) 71 { 72 int x = l, y = r; 73 while (x < y) 74 { 75 if (s[x] != s[y]) return false; 76 x++; y--; 77 } 78 return true; 79 } 80 vector<vector<int>> work(vector<string>& words) 81 { 82 int n = words.size(); 83 Trie* t = new Trie(); 84 vector<vector<int>> res; 85 for (int i = 0; i < n; i++) 86 { 87 t->insert(words[i], i); 88 } 89 vector<int> palin; 90 for (int i = 0; i < n; i++) 91 { 92 int l = words[i].size(); 93 for (int j = 1; j <= l; j++) 94 { 95 if (check(words[i], 0, j-1)) 96 { 97 int id = t->search(words[i], l-1, j); 98 if (id != -1 and id != i) 99 { 100 vector<int> v{id, i}; 101 res.push_back(v); 102 } 103 } 104 if (check(words[i], l-j, l-1)) 105 { 106 int id = t->search(words[i], l-j-1, 0); 107 if (id != -1 and id != i) 108 { 109 vector<int> v{i, id}; 110 res.push_back(v); 111 } 112 } 113 } 114 int id = t->search(words[i], l-1, 0); 115 if (id != -1 and id != i) 116 { 117 vector<int> v{i, id}; 118 res.push_back(v); 119 } 120 } 121 t->del(); 122 return res; 123 } 124 125 vector<vector<int>> palindromePairs(vector<string>& words) 126 { 127 vector<vector<int>> res = work(words); 128 return res; 129 } 130 131 };