298. Binary Tree Longest Consecutive Sequence

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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
2
3
4
5
6
7
1
 \
  3
 / \
2   4
     \
      5

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

1
2
3
4
5
6
7
  2
   \
    3
   /
  2    
 /
1

Longest consecutive sequence path is 2-3,not3-2-1, so return 2.

解法1：

用一个全局变量来存储答案，然后递归遍历数就可以了。
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) {
        if (root == null) {
            return 0;
        }
        helper(root, 1);
        return max;        
    }
    private void helper(TreeNode root, int count) {
        if (root == null) {
            max = Math.max(max, count);
            return;
        }
        max = Math.max(max, count);
        if (root.left != null) {
            if (root.left.val == root.val + 1) {
                helper(root.left, count + 1);
            } else {
                helper(root.left, 1);
            }
        }
        if (root.right != null) {
            if (root.right.val == root.val + 1) {
                helper(root.right, count + 1);
            } else {
                helper(root.right, 1);
            }
        }
    }
}