<p>Given an <code>m x n</code> grid of characters <code>board</code> and a string <code>word</code>, return <code>true</code> <em>if</em> <code>word</code> <em>exists in the grid</em>.</p>

<p>The word can 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.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/04/word2.jpg" style="width: 322px; height: 242px;" />
<pre>
<strong>Input:</strong> board = [[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;,&quot;E&quot;],[&quot;S&quot;,&quot;F&quot;,&quot;C&quot;,&quot;S&quot;],[&quot;A&quot;,&quot;D&quot;,&quot;E&quot;,&quot;E&quot;]], word = &quot;ABCCED&quot;
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/04/word-1.jpg" style="width: 322px; height: 242px;" />
<pre>
<strong>Input:</strong> board = [[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;,&quot;E&quot;],[&quot;S&quot;,&quot;F&quot;,&quot;C&quot;,&quot;S&quot;],[&quot;A&quot;,&quot;D&quot;,&quot;E&quot;,&quot;E&quot;]], word = &quot;SEE&quot;
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/10/15/word3.jpg" style="width: 322px; height: 242px;" />
<pre>
<strong>Input:</strong> board = [[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;,&quot;E&quot;],[&quot;S&quot;,&quot;F&quot;,&quot;C&quot;,&quot;S&quot;],[&quot;A&quot;,&quot;D&quot;,&quot;E&quot;,&quot;E&quot;]], word = &quot;ABCB&quot;
<strong>Output:</strong> false
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>m == board.length</code></li>
	<li><code>n = board[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 6</code></li>
	<li><code>1 &lt;= word.length &lt;= 15</code></li>
	<li><code>board</code> and <code>word</code> consists of only lowercase and uppercase English letters.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> Could you use search pruning to make your solution faster with a larger <code>board</code>?</p>

class Solution {
public:
    
    bool mySearchWord(int nIndex, int nSize,int nX, int nY, int nSizeX, int nSizeY,string sTarget, vector<vector<bool>>visited, vector<vector<char>>& board) {
        if (nY < 0 || nX < 0 || nX > nSizeX-1 || nY > nSizeY-1 || board[nY][nX] != sTarget[nIndex] || visited[nY][nX]) return false;
        if (nIndex == nSize-1) {
            return true;
        }
        visited[nY][nX] = true;
        if (mySearchWord(nIndex + 1, nSize, nX, nY - 1, nSizeX, nSizeY, sTarget,visited, board)) {
            return true;
        }
        if (mySearchWord(nIndex + 1, nSize, nX, nY + 1, nSizeX, nSizeY, sTarget,visited, board)) {
            return true;
        }
        if (mySearchWord(nIndex + 1, nSize, nX-1, nY , nSizeX, nSizeY, sTarget,visited, board)) {
            return true;
        }
        if (mySearchWord(nIndex + 1, nSize, nX + 1, nY, nSizeX, nSizeY, sTarget,visited, board)) {
            return true;
        }
        visited[nY][nX] = false;
    return false;
    }
    bool exist(vector<vector<char>>& board, string word) {
		int nSizeY = board.size(), nSizeX = board[0].size();
		vector<vector<bool>> visited(nSizeY, vector<bool>(nSizeX, false));
        unordered_map<char,int> stat;
        for (int i = 0; i < word.size(); i++) {
            stat[word[i]]++;
        }
        for (int i = 0; i < nSizeY; i++) {
            for (int j = 0;  j< nSizeX; j++) {
                stat[board[i][j]]--;
            }
        }
        unordered_map<char,int>::iterator _it = stat.begin();
        while (_it != stat.end()) {
            if (stat[_it->first] > 0) return false;
            _it++;
        }
        if (stat[word[0]] < stat[word[word.size()-1]]) {
            reverse(word.begin(),word.end());
        }
		for (int i = 0; i < nSizeY; i++) {
			for (int j = 0; j < nSizeX; j++) {
				if (word[0] != board[i][j]) continue;
				if (mySearchWord(0, word.size() , j, i, nSizeX, nSizeY, word, visited, board)) {
					return true;
				}
			}
		}
		return false;
	}
};
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