549. Binary Tree Longest Consecutive Sequence II

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Given a binary tree, you need to find the length of Longest Consecutive Path in Binary Tree.

Especially, this path can be either increasing or decreasing. For example, [1,2,3,4] and [4,3,2,1] are both considered valid, but the path [1,2,4,3] is not valid. On the other hand, the path can be in the child-Parent-child order, where not necessarily be parent-child order.

Example 1:

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Input:
        1
       / \
      2   3
Output: 2
Explanation: The longest consecutive path is [1, 2] or [2, 1].

Example 2:

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Input:
        2
       / \
      1   3
Output: 3
Explanation: The longest consecutive path is [1, 2, 3] or [3, 2, 1].

Note: All the values of tree nodes are in the range of [-1e7, 1e7].

解法1：

要先想想思路
一个node，返回已他为起点的最长的decreasing和increasing
那通过root的最长的一个path一定是increase + decrease - 1, 减1是去掉一次root
C++

1

Java

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int max = 0;
    public int longestConsecutive(TreeNode root) {
        int[] temp = helper(root);
        return max;        
    }
    private int[] helper(TreeNode root) {
        if (root == null) {
            return new int[]{0, 0};
        }
        int increase = 1, decrease = 1;
        if (root.left != null) {
            int[] left = helper(root.left);
            if (root.val == root.left.val + 1) {
                decrease = left[1] + 1;
            } else if (root.val == root.left.val -1) {
                increase = left[0] + 1;
            }
        }
        if (root.right != null) {
            int[] right = helper(root.right);
            if (root.val == root.right.val + 1) {
                decrease = Math.max(decrease, right[1] + 1);
            } else if (root.val == root.right.val -1) {
                increase = Math.max(increase, right[0] + 1);
            }
        }
        max = Math.max(max, decrease + increase -1);
        return new int[]{increase, decrease};
    }
}