Spotting the pattern from the question

Hash map:

• Does any pair in this list add up to the target? (list not sorted): One pass with a set of values seen so far gives O(n).
• Group words that use exactly the same letters: A key built from the letter counts collects each group in one pass.

Two pointers:

• Sorted prices: find two that add up to a budget: Sorted order lets one pointer move in from each end.
• Move all empty slots to the end in place: A write pointer trails a read pointer, O(1) memory.

Sliding window:

• Longest streak of days whose spending stays within a budget: Grow the window, shrink it when the total passes the budget.
• Best total over any k consecutive days: A fixed-size window adds one day and drops one each step.

Binary search:

• Slowest reading pace that still finishes in h hours: 'Finishes in time' flips once as the pace grows, so search the answer.
• Where does a new value go in a sorted list?: Lower-bound search on the sorted list, O(log n).

Heap:

• Keep the k highest scores from a stream: A min-heap of size k drops the smallest each time it overflows.
• Merge many sorted feeds into one: A heap holds the front of each feed, O(N log k).

BFS or DFS:

• Fewest moves between two stations on an unweighted map: BFS reaches nodes in order of distance when every edge counts the same.
• Count the separate land patches on a grid: Each search from an unvisited land cell floods one patch.

Dynamic programming:

• Fewest coins to make an amount: Each amount's best answer reuses smaller amounts' answers.
• Fewest edits to turn one word into another: A table over prefixes of both words, O(m × n).

Backtracking:

• List every order of a small playlist: Every answer must be written out, so choose, explore and undo.
• Place queens on a board so none attack: Try a column per row and undo when a later row has no safe square.