hello-algo/en/docs/chapter_searching/replace_linear_by_hashing.md

Hash optimization strategies

In algorithm problems, we often reduce the time complexity of an algorithm by replacing a linear search with a hash-based search. Let's use an algorithm problem to deepen the understanding.

!!! question

Given an integer array `nums` and a target element `target`, please search for two elements in the array whose "sum" equals `target`, and return their array indices. Any solution is acceptable.

Linear search: trading time for space

Consider traversing through all possible combinations directly. As shown in the figure below, we initiate a nested loop, and in each iteration, we determine whether the sum of the two integers equals target. If so, we return their indices.

The code is shown below:

[file]{two_sum}-[class]{}-[func]{two_sum_brute_force}

This method has a time complexity of O(n^2) and a space complexity of O(1), which can be very time-consuming with large data volumes.

Hash search: trading space for time

Consider using a hash table, where the key-value pairs are the array elements and their indices, respectively. Loop through the array, performing the steps shown in the figure below during each iteration.

1. Check if the number target - nums[i] is in the hash table. If so, directly return the indices of these two elements.
2. Add the key-value pair nums[i] and index i to the hash table.

=== "<1>"

=== "<2>"

=== "<3>"

The implementation code is shown below, requiring only a single loop:

[file]{two_sum}-[class]{}-[func]{two_sum_hash_table}

This method reduces the time complexity from O(n^2) to O(n) by using hash search, significantly enhancing runtime efficiency.

As it requires maintaining an additional hash table, the space complexity is O(n). Nevertheless, this method has a more balanced time-space efficiency overall, making it the optimal solution for this problem.