298. Binary Tree Longest Consecutive Sequence

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).

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.

链接： http://leetcode.com/problems/binary-tree-longest-consecutive-sequence/

题解：

求二叉树中最长连续序列。 题目又比较长， 不过我们可以确定这个最长连续序列肯定是递增的，比如123，或者345。知道这点以后就可以用DFS遍历了。

Time Complexity - O(n)，  Space Complexity - O(n)。

/**
 * 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 max = 0;
    public int longestConsecutive(TreeNode root) {
        if(root == null) {
            return max;
        }
        findLongestConsecutive(root, 0, root.val);
        return max;
    }

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

Reference:

https://leetcode.com/discuss/68723/simple-recursive-dfs-without-global-variable

https://leetcode.com/discuss/66486/c-solution-in-4-lines

https://leetcode.com/discuss/66565/1ms-easy-understand-java-solution-just-traverse-the-tree-once

https://leetcode.com/discuss/68094/dont-understand-what-is-consecutive-sequence

https://leetcode.com/discuss/66646/two-simple-iterative-solutions-bfs-and-dfs

https://leetcode.com/discuss/66548/recursive-solution-bottom-iteration-solution-using-stack

https://leetcode.com/discuss/66584/easy-java-dfs-is-there-better-time-complexity-solution