2. Arrays & Hashing

Arrays and two pointers

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Reading 1

Two indices, one pass

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Two pointers often means left and right moving through an array, or slow and fast for cycles and midpoints.

On a sorted array, left/right can find pairs that sum to a target in O(n) instead of O(n^2).

Always keep bounds clear so you do not walk off the array.

// reverse in place
int l = 0, r = n - 1;
while (l < r) {
  swap(a[l], a[r]);
  l++; r--;
}

Check 2

Open

On a sorted array, two pointers for pair-sum is typically

Fill in 3

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Try it 4

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Watch the array reverse.

main.cpp

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Program input (stdin)

Output will appear here.

Assignment 5

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Read n (1..20), then n integers. Reverse them in place and print separated by spaces.

main.cpp

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