Shared ListNode for linked-list problems.

1/**2 * Shared ListNode for linked-list problems.3 *4 * Learn once, reuse across Blind75 / Interview 150 / Top 100.5 *6 * Compile with a problem file:7 *   javac pkg5leetcode/common/ListNode.java pkg5leetcode/blind75/YourProblem.java8 *   java -cp pkg5leetcode/common;pkg5leetcode/blind75 YourProblem9 *10 * Or study this shape and keep a nested copy for single-file `java YourProblem.java` runs.11 */12public class ListNode {13    public int val;14    public ListNode next;15 16    public ListNode() {}17 18    public ListNode(int val) {19        this.val = val;20    }21 22    public ListNode(int val, ListNode next) {23        this.val = val;24        this.next = next;25    }26 27    /** Build a list from values, e.g. of(1, 2, 3). */28    public static ListNode of(int... values) {29        ListNode dummy = new ListNode(0);30        ListNode cur = dummy;31        for (int v : values) {32            cur.next = new ListNode(v);33            cur = cur.next;34        }35        return dummy.next;36    }37 38    @Override39    public String toString() {40        StringBuilder sb = new StringBuilder();41        ListNode cur = this;42        while (cur != null) {43            if (sb.length() > 0) sb.append(" -> ");44            sb.append(cur.val);45            cur = cur.next;46        }47        return sb.toString();48    }49}