Word Search

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Intuition

DFS/backtracking from each cell. Mark cells visited to avoid reuse. Unmark on backtrack.

Algorithm

1For each cell (i,j): DFS with index into word.
2At each step: check bounds, match, not visited. Mark visited. Recurse in 4 directions. Unmark.

Common Pitfalls

•Mark cell as visited during recursion, unmark after. Check character match before recursing.

Word Search.java

Java

// Approach: Backtracking DFS on grid. For each starting cell try to match the word character by character.
// Time: O(n*m * 4^len) Space: O(len)
class Solution {
    static public boolean isWordExist(char[][] mat, String word) {
        int n = mat.length, m = mat[0].length;
        boolean[][] visited = new boolean[n][m];

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (mat[i][j] == word.charAt(0) && dfs(mat, word, i, j, 0, visited))
                    return true;
            }
        }
        return false;
    }

    private static boolean dfs(char[][] mat, String word, int i, int j, int index, boolean[][] visited) {
        if (index == word.length())
            return true;
        if (i < 0 || i >= mat.length || j < 0 || j >= mat[0].length || visited[i][j] || mat[i][j] != word.charAt(index))
            return false;

        visited[i][j] = true;
        boolean found = dfs(mat, word, i + 1, j, index + 1, visited) ||
                dfs(mat, word, i - 1, j, index + 1, visited) ||
                dfs(mat, word, i, j + 1, index + 1, visited) ||
                dfs(mat, word, i, j - 1, index + 1, visited);

        visited[i][j] = false; // Backtrack
        return found;
    }
}

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