Leetcode 79 - pass online judge - Julia wrote code for Leetcode 213 Word Search II, but the solution failed last 2 test cases.

Leetcode79_wordSearch.cs

public class Solution {
    public bool Exist(char[,] board, string word) {
        if (board == null || 
                board.GetLength(0) == 0 || 
                board.GetLength(1) == 0 || 
                word == null || 
                word.Length == 0)
            {
                return false; 
            }

            int rows = board.GetLength(0);
            int cols = board.GetLength(1);

            for (int row = 0; row < rows; row++)
            {
                for (int col = 0; col < cols; col++)
                {
                    var visit = board[row, col];
                    bool startSearch = visit == word[0];

                    if (!startSearch)
                    {
                        continue; 
                    }

                    var memo = new bool[rows, cols]; 
                    bool foundOne = searchWord(board, word, row, col, 0, memo);

                    if (foundOne)
                    {
                        return true; 
                    }
                }
            }

            return false; 
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="board"></param>
        /// <param name="word"></param>
        /// <param name="startRow"></param>
        /// <param name="startCol"></param>
        /// <returns></returns>
        private static bool searchWord(
            char[,] board, 
            string word, 
            int startRow, 
            int startCol, 
            int index, 
            bool[,] memo)
        {
            int rows = board.GetLength(0);
            int cols = board.GetLength(1);

            if(! isInBoundary(board, startRow, startCol) || memo[startRow,startCol])
            {
                return false;
            }

            var visit = board[startRow, startCol];
            var length = word.Length;

            bool isEqual = index < length && visit == word[index];
            bool isLastChar = index == (length - 1);

            if (!isEqual)
            {
                return false; 
            }

            memo[startRow, startCol] = true; 

            // base case 
            if (isLastChar)
            {
                return true; 
            }

            var nextIndex = index + 1; // bug found through debug - nextIndex++ in four recursive calls
            var result =
                   searchWord(board, word, startRow,     startCol - 1, nextIndex, memo) ||
                   searchWord(board, word, startRow,     startCol + 1, nextIndex, memo) ||
                   searchWord(board, word, startRow - 1, startCol,     nextIndex, memo) ||
                   searchWord(board, word, startRow + 1, startCol,     nextIndex, memo);

            if (!result)
            {
                memo[startRow, startCol] = false;
            }

            return result; 
        }

        private static bool isInBoundary(char[,] board, int startRow, int startCol)
        {
            int rows = board.GetLength(0);
            int cols = board.GetLength(1);

            return  startRow >= 0 && startRow < rows &&
                    startCol >= 0 && startCol < cols;
        }
}