[LeetCode] 543. Diameter of Binary Tree

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 longestpath 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.

题意：计算一个树的最长路线，两个点之间的最长路线

这里有一点说明一下，可能过根节点，也可能不过根节点

          1
         / 
        2   3
       /      
      4   5    

拿上面这个树举个例子，4213这条路线为最长路，过了根节点，

我们现在假设425是最长路线(假设不过根节点的情况)，那么这个时候我们其实可以把2看做新的根节点

什么意思呢，也就是说最长路线一定是某个点的左子树高度+右子树高度，我们把每个点的这个值都算出来，找出最大的即可

class Solution {
    private int max = 0;
    private int countDiameter(TreeNode root) {
        if (root == null)
            return 0;
        return Math.max(countDiameter(root.right), countDiameter(root.left)) + 1;
    }
    public int diameterOfBinaryTree(TreeNode root) {
        if (root == null)
            return 0;
        max = Math.max(max, countDiameter(root.left) + countDiameter(root.right));
        diameterOfBinaryTree(root.left);
        diameterOfBinaryTree(root.right);
        return max;
    }
}