Merge pull request neetcode-gh#97 from Pegasus02K/patch-11

Create 212-Word-Search-II.cpp

Author: neetcode-gh

cpp/212-Word-Search-II.cpp

@@ -0,0 +1,72 @@
+struct TrieNode {
+	TrieNode* children[26];
+	int wordEndCnt;
+	string word;
+
+	TrieNode() : wordEndCnt(0), word("") {
+		for (int i = 0; i < 26; ++i)
+			children[i] = nullptr;
+	}
+};
+
+class Solution {
+public:
+	void addWord(TrieNode* node, const string& word) {
+		// simple trie insertion
+		TrieNode* cur = node;
+
+		int idx;
+		for (int i = 0; i < word.size(); ++i) {
+			idx = word[i] - 'a';
+			if (cur->children[idx] == nullptr)
+				cur->children[idx] = new TrieNode;
+			cur = cur->children[idx];
+		}
+		// increase the word end counter
+		++cur->wordEndCnt;
+		cur->word = word;
+	}
+
+	void solve(vector<vector<char>>& board, int ROWS, int COLS, int r, int c, vector<string>& result, TrieNode* cur) {
+		// boundary check
+		if (r < 0 || r >= ROWS || c < 0 || c >= COLS)
+			return;
+		// visit check
+		if (board[r][c] == '$')
+			return;
+		// existence check 
+		int idx = board[r][c] - 'a';
+		if (cur->children[idx] == nullptr)
+			return;
+		cur = cur->children[idx];
+		if (cur->wordEndCnt > 0) {
+			// found word
+			result.push_back(cur->word);
+			--cur->wordEndCnt;
+		}
+
+		// mark as visited and backtrack
+		char origin = board[r][c];
+		board[r][c] = '$';
+		solve(board, ROWS, COLS, r + 1, c, result, cur);
+		solve(board, ROWS, COLS, r - 1, c, result, cur);
+		solve(board, ROWS, COLS, r, c + 1, result, cur);
+		solve(board, ROWS, COLS, r, c - 1, result, cur);
+		board[r][c] = origin;
+	}
+
+	vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
+		TrieNode* root = new TrieNode;
+		for (auto w : words) {
+			addWord(root, w);
+		}
+		int ROWS = board.size(), COLS = board[0].size();
+
+		vector<string> result;
+		// check every cells
+		for (int i = 0; i < ROWS; ++i)
+			for (int j = 0; j < COLS; ++j)
+				solve(board, ROWS, COLS, i, j, result, root);
+		return result;
+	}
+};