leetcode 212. Word Search II

class Solution {
public:
vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
TrieNode* root = new TrieNode();
for (auto str : words)
insert(root, str);
visited.resize(board.size(), vector<bool>(board[0].size(), false));
for (int row = 0; row < board.size(); ++row)
for (int col = 0; col < board[row].size(); ++col)
backTrack(root, board, row, col, "");
vector<string> ret;
for (auto elem : set_ret)
ret.push_back(elem);
return ret;
}
private:
vector<vector<bool>> visited;
set<string> set_ret;
struct TrieNode {
const static unsigned  LETTERS_NUM = 26;
bool hasString;
struct TrieNode* next[LETTERS_NUM];
TrieNode() : hasString(false) {
for (int i = 0; i < LETTERS_NUM; ++i)
next[i] = NULL;
}
};
void insert(TrieNode* root, string str) {
string::size_type i = 0;
TrieNode* ptr = root;
while (i < str.size()) {
if (!ptr->next[str[i] - 'a'])
ptr->next[str[i] - 'a'] = new TrieNode();
ptr = ptr->next[str[i] - 'a'];
i++;
}
ptr->hasString = true; // 允许空字符串
}
void backTrack(TrieNode* root, vector<vector<char>>& board, int row, int col, string str) {

auto child = root->next[board[row][col] - 'a'];

/*尽量避免这样写, 之前visited[row][col]=true写在第一行，
导致 visited[row][col] = false无法执行 */
if (!child) 	return;

visited[row][col] = true;
str += board[row][col];
if (child->hasString)
set_ret.insert(str);
int dir[4][2] = { { 1, 0 },{ 0, 1 },{ -1, 0 },{ 0, -1 } };
for (int i = 0; i < 4; ++i) {
int r = dir[i][0] + row,
c = dir[i][1] + col;
/* 屡次犯的低级错误
* if ( !visited[r][c] &&r >= 0 && r < board.size()
&& c >= 0 && c < board[r].size())
*/
if (r >= 0 && r < board.size() && c >= 0 && c < board[r].size() && !visited[r][c])
backTrack(child, board, r, c, str);
}
visited[row][col] = false;
}
};