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Word Search II

Tags : string, array, matrix, backtracking, trie, leetcode, cpp, hard

Problem Statement - link #

Given an m x n board of characters and a list of strings words, return all words on the board.

Each word must be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.

Examples #

Example 1:

Input: board = [["o","a","a","n"],["e","t","a","e"],["i","h","k","r"],["i","f","l","v"]], words = ["oath","pea","eat","rain"]
Output: ["eat","oath"]

Example 2:

Input: board = [["a","b"],["c","d"]], words = ["abcb"]
Output: []

Constraints #

Solutions #

class Solution {
public:
  
    bool mnk(vector<vector<char>>& board, string word, int i, int j, int wi){
        
        if( wi == -1) return true;
        
        if( i >= board.size() || j >= board[0].size() || i<0 || j<0|| board[i][j] == '*' || board[i][j] != word[wi]  ) return false;
        
        char curr = board[i][j];
        board[i][j] = '*';
        
        bool ans = ( mnk(board,word,i,j-1,wi-1) || mnk(board,word,i,j+1,wi-1) || mnk(board,word,i-1,j,wi-1) || mnk(board,word,i+1,j,wi-1) );
      
        board[i][j] = curr;
      
        return ans;
        
    }
  
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
      
        vector<string> answer;
        bool yess;
      
        set<char> s;
        for(int i=0; i<words.size(); i++){
            char ch = words[i][words[i].length()-1]; 
            s.insert(ch);
        }
        
        map<char,vector<pair<int,int>>> mp;
        for(int i = 0; i < board.size(); i++){
            for(int j = 0; j < board[0].size(); j++){
                char ch = board[i][j];
                if(s.find(ch) != s.end()){
                    mp[ch].push_back({i,j}); 
                }
            }
        }
        
        for(int i=0 ; i < words.size(); i++){
            string st = words[i];
            char ch = st[st.length()-1];
            vector<pair<int,int>> v = mp[ch];
            for(int j=0; j<v.size(); j++){
                int x = v[j].first;
                int y = v[j].second;
                if(mnk(board, st, x, y, st.length()-1)){
                    answer.push_back(st);
                    break;
                }
            }
        }        

        return answer;
    }
};