牛客NC242 单词搜索【中等递归DFS C++/Java/Go/PHP】

题目

在这里插入图片描述

题目链接：
https://www.nowcoder.com/practice/987f2981769048abaf6180ed63266bb2

思路

递归：以word第一个字符为起点，在矩阵中
递归搜索，检查是否存在完整的word路径，
注意恢复现场，又叫回溯，这是核心

参考答案C++

class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param board string字符串vector
     * @param word string字符串
     * @return bool布尔型
     */
    bool exist(vector<string>& board, string word) {
        //dfs
        int n = board.size();
        int m = board[0].size();

        vector<vector<bool>> visted(n, vector<bool>(m));

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (board[i][j] == word[0]) {
                    if (dfs(board, i, j, word, 0, visted)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }

    bool dfs(vector<string>& board, int i, int j, string word, int idx,
             vector<vector<bool>> visited) {
        if (idx == word.size())
            return true;
        if (i < 0 || i >= board.size() || j < 0 || j >= board[0].size() ||
                visited[i][j] || board[i][j] != word[idx])
            return false;

        visited[i][j] = true;
        //从上下左右搜索
        bool b1 = dfs(board, i - 1, j, word, idx + 1, visited);
        bool b2 = dfs(board, i + 1, j, word, idx + 1, visited);
        bool b3 = dfs(board, i, j - 1, word, idx + 1, visited);
        bool b4 = dfs(board, i, j + 1, word, idx + 1, visited);
        visited[i][j] = false; //恢复现场，又叫回溯

        return b1 || b2 || b3 || b4;
    }
};

参考答案Java

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param board string字符串一维数组
     * @param word string字符串
     * @return bool布尔型
     */
    public boolean exist (String[] board, String word) {
        //dfs
        int n = board.length;
        int m = board[0].length();
        boolean[][] visited = new boolean[n][m];
        for (int i = 0; i < n ; i++) {
            for (int j = 0; j < m ; j++) {
                if (board[i].charAt(j) == word.charAt(0)) {
                    //开始dfs
                    if ( dfs(board, i, j, word, 0, visited)) {
                        return true;
                    }
                }
            }
        }

        return false;
    }

    //当前来到board的i行j列，检查board[i,j]是否等于word[idx]
    public boolean dfs(String[] board, int i, int j, String word, int idx,
                       boolean[][] visited) {
        if (idx == word.length()) return true; //说明在矩阵中找到了单词

        //判断是否越界，是否访问过，检查board[i,j]是否等于word[idx]
        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length() ||
                visited[i][j] || board[i].charAt(j) != word.charAt(idx)) {
            return false;
        }
        visited[i][j] = true;
        boolean b1 = dfs(board, i - 1, j, word, idx + 1, visited);
        boolean b2 =  dfs(board, i + 1, j, word, idx + 1, visited);
        boolean b3 =  dfs(board, i, j - 1, word, idx + 1, visited);
        boolean b4 =  dfs(board, i, j + 1, word, idx + 1, visited);
        visited[i][j] = false; //恢复现场，又叫回溯

        return b1 || b2 || b3 || b4;
    }
}

参考答案Go

package main

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 *
 * @param board string字符串一维数组
 * @param word string字符串
 * @return bool布尔型
 */
func exist(board []string, word string) bool {
	//dfs
	n := len(board)
	m := len(board[0])
	visited := make([][]bool, n)
	for i := 0; i < n; i++ {
		visited[i] = make([]bool, m)
	}

	for i := 0; i < n; i++ {
		for j := 0; j < m; j++ {
			if board[i][j] == word[0] {
				if dfs(board, i, j, word, 0, visited) {
					return true
				}
			}
		}
	}
	return false
}

func dfs(board []string, i, j int, word string, idx int, visited [][]bool) bool {
	if idx == len(word) {
		return true
	}

	if i < 0 || i >= len(board) || j < 0 || j >= len(board[0]) || visited[i][j] || board[i][j] != word[idx] {
		return false
	}

	visited[i][j] = true
	//上下左右搜索
	b1 := dfs(board, i-1, j, word, idx+1, visited)
	b2 := dfs(board, i+1, j, word, idx+1, visited)
	b3 := dfs(board, i, j-1, word, idx+1, visited)
	b4 := dfs(board, i, j+1, word, idx+1, visited)
	visited[i][j] = false //恢复现场，又叫回溯

	return b1 || b2 || b3 || b4
}

参考答案PHP

<?php


/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 * 
 * @param board string字符串一维数组 
 * @param word string字符串 
 * @return bool布尔型
 */
function exist( $board ,  $word )
{
      //dfs
    $n= count($board);
    $m = strlen($board[0]);
    $visited =[];
    for($i=0;$i<$n;$i++){
        for($j=0;$j<$m;$j++){
            $visited[$i][$j]=false;
        }
    }

    for($i=0;$i<$n;$i++){
        for($j=0;$j<$m;$j++){
          if($board[$i][$j] ==$word[0]){
              if(dfs($board,$i,$j,$word,0,$visited)){
                  return true;
              }
          }
        }
    }

    return false;
}


function dfs($board,$i,$j,$word,$idx,$visited){
    if($idx == strlen($word))
        return true;

    if($i<0 || $i>=count($board) || $j<0 || $j>=strlen($board[0]) ||$visited[$i][$j] || $board[$i][$j]!=$word[$idx])
        return false;

    $visited[$i][$j]=true;
    //上下左右四个方向搜索
    $b1 = dfs($board,$i-1,$j,$word,$idx+1,$visited);
    $b2 = dfs($board,$i+1,$j,$word,$idx+1,$visited);
    $b3 = dfs($board,$i,$j-1,$word,$idx+1,$visited);
    $b4 = dfs($board,$i,$j+1,$word,$idx+1,$visited);
    $visited[$i][$j]=false; //恢复现场，又叫回溯
    return $b1||$b2||$b3||$b4;
}