[Locked] Binary Tree Longest Consecutive Sequence

Posted 2020-06-16

Binary Tree Longest Consecutive Sequence

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,not3-2-1, so return 2.

分析：

　　可看作是二分树上的动态规划，自底向上和自顶向下DFS均可

代码： 

int height(TreeNode *node, int &totalmax) {
    if(!node)
        return 0;
    //自底向上DFS
    int leftlength = height(node->left, totalmax), rightlength = height(node->right, totalmax);
    if(node->left && node->left->val - 1 != node->val)
        leftlength = 0;
    if(node->right && node->right->val - 1 != node->val)
        rightlength = 0;
    int nodemax = max(leftlength + 1, rightlength + 1);
    totalmax = max(totalmax, nodemax);
    return nodemax;
}
int path(TreeNode *root) {
    int totalmax = INT_MIN;
    height(root, totalmax);
    return totalmax;
}