595. Binary Tree Longest Consecutive Sequence [LintCode]

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).

Example

For example,

   1
    \
     3
    / \
   2   4
        \
         5

Longest consecutive sequence path is3-4-5, so return3.

   2
    \
     3
    / 
   2    
  / 
 1

Longest consecutive sequence path is2-3,not3-2-1, so return2.

solution 3 是从 246. Binary Tree Path Sum II [LintCode] 中学到的traversal的方法“buffer回看法”。

// solution 1
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param root the root of binary tree
     * @return the length of the longest consecutive sequence path
     */
    public int maxCount = 0;

    public int longestConsecutive(TreeNode root) {
        helper(root);
        return maxCount;
    }

    private int helper(TreeNode node){
        if(node == null) {
            return 0;
        }
        int left = helper(node.left);
        int right = helper(node.right);
        int subtreeMax = 1;
        if(node.left != null && node.left.val == node.val + 1) {
            subtreeMax = Math.max(subtreeMax, left + 1);
        }
        if(node.right != null && node.right.val == node.val + 1) {
            subtreeMax = Math.max(subtreeMax, right + 1);
        }
        if(subtreeMax > maxCount) {
            maxCount = subtreeMax;
        }
        return subtreeMax;
    }
}

// solution 2
public class Solution {
    /**
     * @param root the root of binary tree
     * @return the length of the longest consecutive sequence path
     */

    public int longestConsecutive(TreeNode root) {
        return helper(root, null, 0);
    }

    private int helper(TreeNode root, TreeNode parent, int lengthWithoutRoot) {
        if (root == null) {
            return 0;
        }
        int length = (parent != null && parent.val + 1 == root.val)
            ? lengthWithoutRoot + 1
            : 1;
        int left = helper(root.left, root, length);
        int right = helper(root.right, root, length);
        return Math.max(length, Math.max(left, right));
    }
}

// solution 3
class Solution {
    private int maxCount;
    public int longestConsecutive(TreeNode root) {
        this.maxCount = 0;
        List<Integer> buffer = new ArrayList<Integer>();
        helper(root, buffer);
        return this.maxCount;
    }

    private void helper(TreeNode node, List<Integer> buffer) {
        if (node == null) {
            return;
        }
        buffer.add(node.val);
        int count = 1;
        for (int i = buffer.size() - 1; i > 0; i--) {
          if (buffer.get(i) != buffer.get(i - 1) + 1) {
            break;
          } 
          count++;
        }
        this.maxCount = Math.max(this.maxCount, count);
        helper(node.left, buffer);
        helper(node.right, buffer);
        buffer.remove(buffer.size() - 1);
  }
}