Nearest multiple of 10

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Time: O(1)

Space: O(1)

Intuition

Round a number string to nearest multiple of 10. Check last digit and round up or down.

Algorithm

1If last digit < 5: set to 0. If >= 5: set to 0 and carry +1 through preceding digits.

Common Pitfalls

•Handle carry propagation. Edge case: all 9s become "100...0". String arithmetic.

Nearest multiple of 10.java

Java

// Approach: Find floor and ceiling multiples of 10. Return whichever is closer; tie goes to ceiling.
// Time: O(1) Space: O(1)
class Solution {

    String roundToNearest(String str) {
        StringBuilder ans = new StringBuilder(str);
        int n = str.length();
        int t = (str.charAt(n - 1) - '0') % 10;
        if (t <= 5)
            ans.setCharAt(n - 1, '0');
        else {
            ans.setCharAt(n - 1, '0');
            for (int i = n - 2; i >= 0; i--) {
                char ch = ans.charAt(i);
                if (ch == '9')
                    ans.setCharAt(i, '0');
                else {
                    ans.setCharAt(i, (char) (ch + 1));
                    break;
                }
            }
        }
        return ans.toString();
    }
}

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