0092MediumString

Salvage the Leading Number

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Trains the technique from

This is an original problem, written from a brief that listed the technique, the difficulty, the topics, the function shape and the input bounds — none of that problem's wording, examples, hints or editorials. The link is there so you can map your practice onto the standard set.

Same function shape, different story and different numbers.

A telemetry importer receives one text field per row and has to salvage a whole number from the front of it. Implement the salvage routine, which behaves like this:

Step over every space character sitting at the very front of field.
If the character now under the cursor is + or -, consume that one character and let it decide the sign. Only the first such character counts; a second one is not part of a number.
Consume the run of decimal digits that follows, stopping at the first character that is not a digit or at the end of the field. Leading zeros are harmless. Everything from the stopping point onward, whatever it is, is discarded.
If step 3 consumed no digits at all, the salvaged value is 0.
The importer stores the result in a 32-bit signed register whose range is -2147483648 through 2147483647 inclusive. A salvaged value below that range is reported as -2147483648, and a value above it as 2147483647.

Return the value the importer stores. Do not treat any character other than a space as padding.

Examples

Example 1

Input: field = " -0073abc"
Output: -73

Two spaces are stepped over, the minus fixes the sign, then 0073 is consumed and abc is discarded.

Example 2

Input: field = "8x7"
Output: 8

The digit run ends at x, so the trailing 7 never gets looked at.

Example 3

Input: field = ".55"
Output: 0

A period is neither a space nor a sign nor a digit, so the cursor stops before any digit is consumed.

Example 4

Input: field = "-99999999999"
Output: -2147483648

The salvaged value sits below the register floor and is reported as the floor.

Constraints

0 <= field.length <= 200
field contains only English letters (upper and lower case), digits, ' ', '+', '-' and '.'

The signature

The editor is preloaded with this. It matches the parent problem's shape, so a solution that works here transfers to a judge unchanged.

Python

def salvage_number(field: str) -> int:

Java

public int salvageNumber(String field)

The intended approach

Time O(n)Space O(1)

Walk the field once with an explicit cursor through three phases: spaces, an optional sign, then digits. The register bound is checked while each digit is folded in, so an oversized run is cut short at the boundary the moment it crosses it rather than being accumulated first. A digit counter distinguishes a genuine zero from a field where no digit was ever consumed.

Pythonentry point: salvage_number

def salvage_number(field):
    floor_value = -2147483648
    ceil_value = 2147483647
    n = len(field)
    i = 0
    while i < n and field[i] == " ":
        i += 1
    negative = False
    if i < n and (field[i] == "+" or field[i] == "-"):
        negative = field[i] == "-"
        i += 1
    magnitude = 0
    digits = 0
    while i < n and "0" <= field[i] <= "9":
        magnitude = magnitude * 10 + (ord(field[i]) - ord("0"))
        digits += 1
        if negative:
            if -magnitude < floor_value:
                return floor_value
        else:
            if magnitude > ceil_value:
                return ceil_value
        i += 1
    if digits == 0:
        return 0
    return -magnitude if negative else magnitude

The obvious version

Anchor a regular expression at the start of the field, convert the captured digit run in one go, then clamp.

This is not filler. It was written independently of the fast solution and the two were run against thousands of random inputs to confirm they agree — which is how the answer above is known to be right. Read it when the intended approach feels like a rabbit pulled out of a hat.

Python

def bf_salvage_number(field):
    import re
    match = re.match(r"[ ]*([+-]?)([0-9]+)", field)
    if match is None:
        return 0
    value = int(match.group(2))
    if match.group(1) == "-":
        value = -value
    if value < -2147483648:
        return -2147483648
    if value > 2147483647:
        return 2147483647
    return value

How this problem was checked

Reference solution executed against 15 test cases — 15 passed
4 worked examples confirmed by running the code
0 eight-word sequences shared with the parent problem's text
Agrees with an independent brute-force implementation on random inputs
Test set proven to catch deliberately broken solutions
Reviewed by a separate quality pass
Function arity matches the parent problem's

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