Leetcode: Binary Tree Longest Consecutive Sequence

Posted on July 25, 2018September 23, 2019 by braindenny

Binary Tree Longest Consecutive Sequence

Similar Problems:

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 1:

Input:

   1
    \
     3
    / \
   2   4
        \
         5

Output: 3

Explanation: Longest consecutive sequence path is 3-4-5, so return 3.

Example 2:

Input:

   2
    \
     3
    / 
   2    
  / 
 1

Output: 2 

Explanation: Longest consecutive sequence path is 2-3, not 3-2-1, so return 2.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

Solution:

// https://code.dennyzhang.com/binary-tree-longest-consecutive-sequence
// Basic Ideas: pre-order
//
// Complexity: Time O(n), Space O(1)
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func preOrder(root *TreeNode, maxCnt *int) int {
    if root == nil { return 0 }
    res := 1
    var count int
    if root.Left != nil {
        count = 1
        v := preOrder(root.Left, maxCnt)
        if root.Val + 1 == root.Left.Val {  count = v + 1 }
        if count > res { res = count }
    }
    if root.Right != nil {
        count = 1
        v := preOrder(root.Right, maxCnt)
        if root.Val + 1 == root.Right.Val {  count = v + 1 }
        if count > res { res = count }
    }
    if res > *maxCnt { *maxCnt = res }
    return res
}

func longestConsecutive(root *TreeNode) int {
    maxCnt := 0
    preOrder(root, &maxCnt)
    return maxCnt
}

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