Happy Condition First Before Failure

We want to check the happy condition first before failure.

For example, in the case of WordSearch, with the following implementation:

class App
    def searchWord(self, board, row, col, word, index, visited):
		# Invalid check first
        if not self.isValidCell(board, row, col, visited): 
            return False

		# Happy check after
        if index >= len(word):
            return True

		currentLetter = board[row][col] 
        if currentLetter != word[index]: return False

		# ...

In here, our algorithm will only return true if it passes all the tests. That means we only return True if index reaches to the end of word.

However, what happen if we went through the word already but the cell is invalid? For example:

In here we're following this path: A -> B -> C -> F -> J -> H -> G. Let's say after G, we found the word we're looking for. However if we go after G, the algorithm will return False since it's an invalid index.

Therefore, we need to swap the happy scenario on top first:

class App
    def searchWord(self, board, row, col, word, index, visited):
		# NOTE: Happy check here first
        if index >= len(word):
            return True
            
		# Invalid check after
        if not self.isValidCell(board, row, col, visited): 
            return False

		currentLetter = board[row][col] 
        if currentLetter != word[index]: return False

		# ...