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One optimization technique to improve the performance of the word search solver is to apply ____ to eliminate unnecessary search branches. By analyzing the word and grid beforehand, we can identify word patterns that are impossible to exist. For example, if the grid does not contain any of the characters in the word, we can immediately return false without performing the search. Similarly, if the word contains duplicate characters and the grid does not have multiple occurrences of those characters, we can also return false.

This approach helps to reduce the search space and improves the efficiency of the word search solver.