Medium
簡直是教科書般的DFS+Backtracking. 幾個地方可能會出錯：
- 如何開始搜索， 這里是看第一個單詞符合就開始搜索，如果返回true就說明找到了一個，找到了就返回true
- 找到一個，要么就是或 || 找到了返回true, 要么就是for循環一個一個試(permutation)，這里是或.這里的backtracking其實就是visited[i][j]最后變回false;
class Solution {
public boolean exist(char[][] board, String word) {
if (board == null || board.length == 0 || board[0].length == 0){
return false;
}
boolean[][] visited = new boolean[board.length][board[0].length];
for (int i = 0; i < board.length; i++){
for (int j = 0; j < board[0].length; j++){
if (board[i][j] == word.charAt(0) && dfsHelper(board, i, j, word, 0, visited)){
return true;
}
}
}
return false;
}
private boolean dfsHelper(char[][] board, int i, int j, String word, int charIndex, boolean[][] visited){
if (charIndex == word.length()){
return true;
}
if (i < 0 || i >= board.length || j < 0 || j >= board[0].length || visited[i][j]
|| word.charAt(charIndex) != board[i][j]){
return false;
}
visited[i][j] = true;
if (dfsHelper(board, i + 1, j, word, charIndex + 1, visited) || dfsHelper(board, i - 1, j, word, charIndex + 1, visited) || dfsHelper(board, i, j + 1, word, charIndex + 1, visited) || dfsHelper(board, i, j - 1, word, charIndex + 1, visited)){
return true;
}
visited[i][j] = false;
return false;
}
}
