LeetCode(543)——Diameter of Binary Tree

題目：

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree

          1
         / \
        2   3
       / \     
      4   5    

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.

 

求直徑，其實就是求任意兩點間的最大距離。由於是樹，兩個節點肯定有是公共祖先的，此時兩個節點間的最大長度就是求公共祖先左右子樹的最大深度，加在一起正好是要求的距離。

 

AC:

    private int maxDiameter = 0;
    
    public int diameterOfBinaryTree(TreeNode root) {
        dfs(root);
        return maxDiameter;
    }
    
    private int dfs(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        int leftHeigth = dfs(root.left);
        int rightHeigth = dfs(root.right);
        
        maxDiameter = Math.max(leftHeigth + rightHeigth, maxDiameter);
        
        return Math.max(leftHeigth, rightHeigth) + 1;
    }