Combination Sum

Given an array of distinct integers candidates and a target, return a list of all unique combinations where the chosen numbers sum to target.

Key Rule: The same number may be chosen from candidates an unlimited number of times.

Sort the candidates first. Then backtrack(start, remaining) tries each candidate from start onwards:

1. Too big? If candidates[i] > remaining, break — the list is sorted, so every candidate after it is too big as well.
2. Choose it: append it and subtract it from remaining.
3. Explore with backtrack(i, …) — the same i, because a number may be used again.
4. Unchoose: pop it before trying the next candidate.

When remaining reaches 0 the path is a combination: record it and return.

Passing i rather than 0 is what keeps each combination unique: a call never goes back to a smaller candidate, so [2, 2, 3] is built once and [3, 2, 2] never is.

Every node is one call to backtrack. Its label is the candidate that call added, and the number under it is what is still left to make — a node reading 0 is a combination.

A red node is the one candidate a call looked at and rejected with break: its number goes negative. The candidates after it are never looked at, so they are not drawn at all — that missing part of the tree is the saving. Try Unsorted to see why the sort has to come first.