最长连续序列

题目

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).

样例

  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

代码实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int longest;
    /**
     * @param root the root of binary tree
     * @return the length of the longest consecutive sequence path
     */
    public int longestConsecutive(TreeNode root) {
        longest = 0;
        helper(root);
        return longest;
    }
    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = helper(root.left);
        int right = helper(root.right);
        // at least we have root 
        int subtreeLongest = 1; 
        if (root.left != null && root.val + 1 == root.left.val) {
            subtreeLongest = Math.max(subtreeLongest, left + 1);
        }
        if (root.right != null && root.val + 1 == root.right.val) {
            subtreeLongest = Math.max(subtreeLongest, right + 1);
        }
        
        if (subtreeLongest > longest) {
            longest = subtreeLongest;
        }
        return subtreeLongest; 
    }
}