Plus One Linked List

Question (LC.369)

Given a non-negative integer represented as non-empty a singly linked list of digits, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list.

Analysis

The head node might get modified. Dummy head is needed.

Code

public ListNode plusOne(ListNode head) {
    // increment j if less than 9
    // increment i if equal to 9, and zero the rest
    ListNode dummyNode = new ListNode(0);
    dummyNode.next = head;
    ListNode leastSig = dummyNode; // if less than 9
    ListNode mostSig = dummyNode; // if carry is necessary
    // locate leastSig and mostSig
    while (leastSig.next != null) {
        leastSig = leastSig.next;
        if (leastSig.val < 9) mostSig = leastSig;
    }
    // decide which one to increment
    if (leastSig.val < 9) {
        leastSig.val ++;
    } else {
        mostSig.val++;

        mostSig = mostSig.next;
        while (mostSig != null) {
            mostSig.val = 0;
            mostSig = mostSig.next;
        }
    }
    // decide which one to return
    if (dummyNode.val == 1)
        return dummyNode;
    return head;
}