Implement Atoi

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Time: O(n)

Space: O(1)

Intuition

Parse integer from string: skip whitespace, handle sign, parse digits, clamp to INT range.

Algorithm

1Skip leading whitespace.
2Check sign (+ or -).
3Parse digits: result = result*10 + digit. Stop at non-digit.
4Clamp: if > INT_MAX return INT_MAX; if < INT_MIN return INT_MIN.

Common Pitfalls

•Overflow during parsing: check before multiplying. Empty string or all-whitespace returns 0.

Implement Atoi.java

Java

// Approach: Handle leading spaces, optional sign, then parse digits. Handle overflow by clamping to INT bounds.
// Time: O(n) Space: O(1)

class Solution {

    public int myAtoi(String s) {
        int i = 0;
        while (i < s.length() && s.charAt(i) == ' ') {
            i++;
        }

        int sign = 1;
        if (i < s.length() && (s.charAt(i) == '+' || s.charAt(i) == '-')) {
            if (s.charAt(i) == '-') {
                sign = -1;
            }

            i++;
        }

        long result = 0;
        while (i < s.length() && Character.isDigit(s.charAt(i))) {
            result = result * 10 + (s.charAt(i) - '0');

            if (result * sign > Integer.MAX_VALUE) {
                return Integer.MAX_VALUE;
            }

            if (result * sign < Integer.MIN_VALUE) {
                return Integer.MIN_VALUE;
            }

            i++;
        }

        return (int) (result * sign);
    }
}

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