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298. Binary Tree Longest Consecutive Sequence

荊兆峰 / 3061人閱讀

摘要：題目解答分治，一種不帶返回值，但需要用全局變量，一種帶返回值，不用全局變量有全局變量

題目：
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,not3-2-1, so return 2.

解答：
分治，一種不帶返回值，但需要用全局變量，一種帶返回值，不用全局變量：
1.

//有全局變量
    private int max = 0;
    
    public void Helper(TreeNode root, int target, int count) {
        if (root == null) return;
        if (root.val == target) {
            count++;
        } else {
            count = 1;
        }
        max = Math.max(max, count);
        Helper(root.left, root.val + 1, count);
        Helper(root.right, root.val + 1, count);
    }
    
    public int longestConsecutive(TreeNode root) {
        if (root == null) return 0;
        Helper(root, root.val + 1, 1);
        return max;
    }

2.

public int Helper(TreeNode root, int target, int count) {
    if (root == null) return 1;
    count = root.val == target ? count + 1 : 1;
    int left = Helper(root.left, root.val + 1, count);
    int right = Helper(root.right, root.val + 1, count);
    
    return Math.max(Math.max(left, right), count);
}

public int longestConsecutive(TreeNode root) {
    if (root == null) return 0;
    return Math.max(Helper(root.left, root.val + 1, 1), Helper(root.right, root.val + 1, 1));
}

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