LeetCode 题解(281) ：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,not 3-2-1, so return 2.

题解：

递归。思路很直观。

C++版：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxL = 0;
    
    int longestConsecutive(TreeNode* root) {
        if(root == NULL)
            return 0;
        int l = longest(root);
        return max(l, maxL);
    }
    
    int longest(TreeNode* root) {
        if(root->left == NULL && root->right == NULL) {
            if(maxL == 0)
                maxL = 1;
            return 1;
        }
        int l = 1;
        if(root->left != NULL) {
            int ret = longest(root->left);
            if(ret > maxL)
                maxL = ret;
            if(root->val + 1 == root->left->val)
                l = max(l, ret + 1);
        }
        if(root->right != NULL) {
            int ret = longest(root->right);
            if(ret > maxL)
                maxL = ret;
            if(root->val + 1 == root->right->val)
                l = max(l, ret + 1);
        }
        return l;
    }
};

Java版：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxL = 0;
    
    public int longestConsecutive(TreeNode root) {
        if(root == null)
            return 0;
        int l = longest(root);
        return Math.max(l, maxL);
    }
    
    private int longest(TreeNode root) {
        if(root.left == null && root.right == null) {
            if(maxL == 0)
                maxL = 1;
            return 1;
        }
        int l = 1;
        if(root.left != null) {
            int ret = longest(root.left);
            if(ret > maxL)
                maxL = ret;
            if(root.val + 1 == root.left.val)
                l = Math.max(l, ret + 1);
        }
        if(root.right != null) {
            int ret = longest(root.right);
            if(ret > maxL)
                maxL = ret;
            if(root.val + 1 == root.right.val)
                l = Math.max(l, ret + 1);
        }
        return l;
    }
}