二叉树最长距离

tag: 二叉树

思路:

最长距离一定是两个叶子节点之间的距离

＝> 两个叶子节点必定以某个节点为根节点

＝> 因此用DFS思路自底向上计算经过每一个节点的最长距离，取其最大值

以每个节点为根节点的最长距离 ＝ 左子树的高度＋右子树的高度

package com.zhaochao.tree;

/**
 * Created by zhaochao on 17/1/25.
 */
public class MaxDistanceOfBT {

    int max = 0;
    public int maxDistance(TreeNode root) {
        if(root == null) {
            return 0;
        }
        if(root.left == null && root.right == null) {
            return 0;
        }
        //双枝
        max = Math.max(getHeight(root.left) + getHeight(root.right),max);
        //返回单枝
        return Math.max(getHeight(root.left),getHeight(root.right));
    }

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

    public static void main(String[] args) {

        TreeNode root = new TreeNode(0);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(3);
        TreeNode node5 = new TreeNode(3);
        TreeNode node6 = new TreeNode(3);
        TreeNode node7 = new TreeNode(3);
        TreeNode node8 = new TreeNode(3);
        TreeNode node9 = new TreeNode(3);

        root.left = node1;
        root.right = node2;
        node1.left = node3;
        node3.left = node8;
        node8.left = node9;
        node1.right = node5;
        node5.right = node6;
        node2.right = node4;

        MaxDistanceOfBT test = new MaxDistanceOfBT();

        test.maxDistance(root);
        System.out.println(test.max);

    }

}