Foundations

Study note from the Foundations pattern set — Java templates, complexity analysis and the interview traps that come with it.

Before solving tree problems, ensure you are comfortable with: - The Node Class: How to define class TreeNode { int val; TreeNode left, right; }…

Build a result array by repeating the original sequence once in the first half and once in the second half.

Track the lowest price seen so far; the best profit ending on each day is price today - min price so far.

answer[i] = (product of all elements to the left) × (product of all elements to the right); build prefix and suffix without using division.

The answer lies in [1, n] for array length n; place each valid number x at index x - 1, then scan for the first mismatch.