LESSON 376 OF 410 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: head v4 — Negative n drops the tail in Java

Practice

Same signature as v2/v3, but now n may be negative. Real head -n -K prints all lines except the last K. So when n < 0 return all lines except the last |n|; when n ≥ 0 behave exactly like v3 (return the first n, capped at the input length). If |n| is at least the number of lines, nothing is left, so return the empty string.

Function signature:

public String head(List<String> lines, int n)

Example (negative n):

Input: lines = ["a", "b", "c", "d", "e"], n = -2

Output: "a\nb\nc"

When printed:
a
b
c

Java doesn't auto-handle negative indices. Branch on the sign of n: when n < 0, the end index is lines.size() + n; otherwise it's min(n, lines.size()). Don't forget Math.max(0, ...) for the negative branch in case |n| exceeds the list length.

int end;
if (n < 0) {
    end = Math.max(0, lines.size() + n);
} else {
    end = Math.min(n, lines.size());
}
return String.join("\n", lines.subList(0, end));

The full solution:

class Solution {
    public String head(List<String> lines, int n) {
        int end;
        if (n < 0) {
            end = Math.max(0, lines.size() + n);
        } else {
            end = Math.min(n, lines.size());
        }
        return String.join("\n", lines.subList(0, end));
    }
}

STEP 1 OF 5

Guided

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CLI Tool: head v4 — Negative n drops the tail in Java

Head V4 in Java