Binary Tree Longest Consecutive Sequence

Analysis

这个结果是用Divid & Conquer 做的

1. Divid： tree->left child and right child

2. Conquer: when it is null, return 0

3. Merge：如果是sequence， 则现在的sequence的长度加一， 比较现在的subtree最长序列和已知序列，保留长的。

可以好好看看怎么不用全局变量完成题目

/**
 * 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 class ResultType {
        int maxInSubtree;
        int maxFromRoot;
        public ResultType (int maxInSubtree, int maxFromRoot) {
            this.maxInSubtree = maxInSubtree;
            this.maxFromRoot = maxFromRoot;
        }
    }
    public int longestConsecutive(TreeNode root) {
        // Write your code here
        return helper(root).maxInSubtree;
    }
    private ResultType helper(TreeNode root) {
        if (root == null) {
            return new ResultType(0, 0);
        }
        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        
        //System.out.println(root.val);
        ResultType result = new ResultType(0, 1);
        if (root.left != null && root.val + 1 == root.left.val ) {
            result.maxFromRoot = Math.max(result.maxFromRoot, left.maxFromRoot + 1);
        }
        if (root.right != null && root.val + 1 == root.right.val) {
            result.maxFromRoot = Math.max(result.maxFromRoot, right.maxFromRoot + 1);
        }
        result.maxInSubtree = Math.max(result.maxFromRoot, 
                                        Math.max(left.maxInSubtree, right.maxInSubtree));
        return result;
    }
}