Decode Ways: the idea to remember.

The core insight

A nonzero final digit can stand alone; a final pair from 10 to 26 can extend the prefix two positions earlier.

Why it works

Every decoding ends in exactly one or two digits, and valid endpoint checks cover both disjoint possibilities.

The mistake to avoid

Zero cannot be decoded alone. A pair such as 06 is invalid even though its numeric value is six.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

int numDecodings(String s) {
    int two = 1, one = s.charAt(0) == '0' ? 0 : 1;
    for (int i = 1; i < s.length(); i++) {
        int current = s.charAt(i) == '0' ? 0 : one;
        int pair = (s.charAt(i - 1) - '0') * 10 + s.charAt(i) - '0';
        if (pair >= 10 && pair <= 26) current += two;
        two = one; one = current;
    }
    return one;
}

Make the next connection