lintcode614- Binary Tree Longest Consecutive Sequence II- medium

Posted 2020-10-10 jasminemzy

Given a binary tree, find the length of the longest consecutive sequence path.
The path could be start and end at any node in the tree

Example

    1
   /   2   0
 /
3

Return 4 // 0-1-2-3

分治法。和前一题根本思路一致，判断root.val和左右节点的val是否连续，以确定能否完成接龙。只是本题要记录的变为从root开始向下的最长increase序列和最长decrease序列！于是每次就能搭出从左到右的，最长增长序列与最长减小序列，与全局变量对比，记录。

 

/**
 * 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
     */
    
    private int longestConsec = Integer.MIN_VALUE;
    
    private class ResultType{
        public int incrs;
        public int decrs;
        public ResultType(int incrs, int decrs) {
            this.incrs = incrs;
            this.decrs = decrs;
        }
    }
    
    public int longestConsecutive2(TreeNode root) {
        // write your code here
        if (root == null) {
            return 0;
        }
        helper(root);
        return longestConsec;
    }
    
    //返回从root向下到叶子节点可以的最长的递增consecutive和递减consecutive
    public ResultType helper(TreeNode root) {
        
        if (root == null) {
            return new ResultType(0, 0);
        }
        
        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        
        int crtLeftIncrs = 1;
        int crtLeftDecrs = 1;
        int crtRightIncrs = 1;
        int crtRightDecrs = 1;
        
        if (root.left != null && root.left.val + 1 == root.val) {
            crtLeftDecrs = 1 + left.decrs;
        }
        
        if (root.left != null && root.left.val - 1 == root.val) {
            crtLeftIncrs = 1 + left.incrs;
        }
        
        if (root.right != null && root.right.val + 1 == root.val) {
            crtRightDecrs = 1 + right.decrs;
        }
        
        if (root.right != null && root.right.val - 1 == root.val) {
            crtRightIncrs = 1 + right.incrs;
        }
        
        // check the longest
        if (crtLeftIncrs + crtRightDecrs - 1 > longestConsec) {
            longestConsec = crtLeftIncrs + crtRightDecrs - 1;
        }
        
        if (crtLeftDecrs + crtRightIncrs - 1 > longestConsec) {
            longestConsec = crtLeftDecrs + crtRightIncrs - 1;
        }
        
        return new ResultType(Math.max(crtLeftIncrs, crtRightIncrs), Math.max(crtLeftDecrs, crtRightDecrs));
    }
}