The two readings that add up
A reconciliation tool has a sorted column of amounts and a difference to explain. It looks for the two amounts that together account for it.
- The amounts arrive sorted ascending.
- Return the two amounts, smaller first.
- If several pairs work, return the one with the smallest first amount.
- If no pair adds up, return an empty list.
- An amount cannot pair with itself — the two have to be at different positions.
PairSummingTo(amounts: list<int>, target: int) → list<int>
C# needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
public List<int> PairSummingTo(List<int> amounts, int target) {
}Worked examples
| Call | Result |
|---|---|
PairSummingTo(new List<int> { 1, 2, 4, 7, 11 }, 9) | new List<int> { 2, 7 } |
PairSummingTo(new List<int> { 1, 2, 3, 4 }, 5) | new List<int> { 1, 4 } |
PairSummingTo(new List<int> { 1, 2, 3 }, 100) | new List<int> { } |
PairSummingTo(new List<int> { }, 3) | new List<int> { } |
Hint
Sorted input means you can start at both ends. If the two ends add up to too much, the right end is too big; if too little, the left end is too small.
Reference solution in C#
public List<int> PairSummingTo(List<int> amounts, int target) {
int i = 0, j = amounts.Count - 1;
while (i < j) {
int sum = amounts[i] + amounts[j];
if (sum == target) return new List<int> { amounts[i], amounts[j] };
if (sum < target) i++;
else j--;
}
return new List<int>();
}