BinaryTreeLongestConsecutiveSequence.java

package leetcode;

/**
 * Project Name : Leetcode
 * Package Name : leetcode
 * File Name : BinaryTreeLongestConsecutiveSequence
 * Creator : Edward
 * Date : Aug, 2017
 * Description : 298. Binary Tree Longest Consecutive Sequence
 */
public class BinaryTreeLongestConsecutiveSequence {

    /**
     * Given a binary tree, find the length of the longest consecutive sequence path.

     The path refers to any sequence of nodes from some starting node to any node
     in the tree along the parent-child connections. The longest consecutive path need to
     be from parent to child (cannot be the reverse).

     For example,
      1
      \
       3
      / \
     2   4
          \
           5
     Longest consecutive sequence path is 3-4-5, so return 3.
        2
         \
         3
        /
       2
      /
     1
     Longest consecutive sequence path is 2-3,not3-2-1, so return 2.

     time : O(n);
     space : O(n);
     * @param root
     * @return
     */

    private int res = 0;

    public int longestConsecutive(TreeNode root) {
        if (root == null) return 0;
        helper(root, 0, root.val);
        return res;
    }

    public void helper(TreeNode root, int max, int target) {
        if (root == null) return;
        if (root.val == target) {
            max++;
        } else max = 1;
        res = Math.max(res, max);
        helper(root.left, max, root.val + 1);
        helper(root.right, max, root.val + 1);
    }

}