二叉树的直径

题目

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.

简洁代码

  public int diameterOfBinaryTree(TreeNode root) {
        if(root == null) return 0;
        //左右子树的深度之和
        int diameter = getDepth(root.left) + getDepth(root.right);
        //返回左子树、右子树、左右子树的深度之和中的最大值
        return Math.max(diameter,Math.max(diameterOfBinaryTree(root.left),diameterOfBinaryTree(root.right)));
    }
    //求树的深度
    public int getDepth(TreeNode root){
        if(root == null) return 0;
        return Math.max(getDepth(root.left),getDepth(root.right)) + 1;
    }

过程分析

public class TreeDiemiter {

    static class TreeNode{
        TreeNode left = null;
        TreeNode right = null;
        int val = 0;

        public TreeNode(int val) {
            this.val = val;
        }
    }

    public int diameterOfBinaryTree(TreeNode root) {
        if(root == null) return 0;
        //左右子树的深度之和
        int diameter = getDepth(root.left) + getDepth(root.right);
        //返回左子树、右子树、左右子树的深度之和中的最大值
        return Math.max(diameter,Math.max(diameterOfBinaryTree(root.left),diameterOfBinaryTree(root.right)));
    }
    //求树的深度
    public int getDepth(TreeNode root){
        if(root == null) return 0;
        return Math.max(getDepth(root.left),getDepth(root.right)) + 1;
    }
    /*另外一种写法求树的深度
      public int TreeDepth(TreeNode root) {
         if(root == null) return 0;
        int leftDepth = TreeDepth(root.left);
        int rightDepth = TreeDepth(root.right);
        return leftDepth > rightDepth? leftDepth + 1:rightDepth + 1;//这里要加1的原因是深度数的是节点的个数
    }*/

    public static void main(String[] args){
        TreeDiemiter.TreeNode treeNode = new TreeDiemiter.TreeNode(1);
        treeNode.left = new TreeDiemiter.TreeNode(2);
        treeNode.right = new TreeDiemiter.TreeNode(3);
        treeNode.left.left = new TreeDiemiter.TreeNode(4);
        treeNode.left.right = new TreeDiemiter.TreeNode(5);
        TreeDiemiter treeDiemiter = new TreeDiemiter();
        int res = treeDiemiter.diameterOfBinaryTree(treeNode);
        System.out.println(res);

    }
}

public class TreeDiemiter {

    static class TreeNode{
        TreeNode left = null;
        TreeNode right = null;
        int val = 0;

        public TreeNode(int val) {
            this.val = val;
        }
    }

    public int diameterOfBinaryTree(TreeNode root) {
        System.out.println("=====================");
        if(root == null) return 0;
        //左右子树的深度之和
        int leftDepth = getDepth(root.left);
        System.out.println("leftDepth:" + leftDepth);
        int rightDepth = getDepth(root.right);

        System.out.println("rightDepth:" + rightDepth);
        int diameter = leftDepth + rightDepth;
        System.out.println("diameter:" + diameter);
        //返回左子树、右子树、左右子树的深度之和中的最大值
        return Math.max(diameter,Math.max(diameterOfBinaryTree(root.left),diameterOfBinaryTree(root.right)));
    }
    //求树的深度
    public int getDepth(TreeNode root){
        if(root == null) return 0;
        return Math.max(getDepth(root.left),getDepth(root.right)) + 1;
    }

    public static void main(String[] args){
        TreeDiemiter.TreeNode treeNode = new TreeDiemiter.TreeNode(1);
        treeNode.left = new TreeDiemiter.TreeNode(2);
        treeNode.right = new TreeDiemiter.TreeNode(3);
        treeNode.left.left = new TreeDiemiter.TreeNode(4);
        treeNode.left.right = new TreeDiemiter.TreeNode(5);
        TreeDiemiter treeDiemiter = new TreeDiemiter();
        int res = treeDiemiter.diameterOfBinaryTree(treeNode);
        System.out.println(res);

    }
}

=====================
leftDepth:2
rightDepth:1
diameter:3
=====================
leftDepth:1
rightDepth:1
diameter:2
=====================
leftDepth:0
rightDepth:0
diameter:0
=====================
=====================
=====================
leftDepth:0
rightDepth:0
diameter:0
=====================
=====================
=====================
leftDepth:0
rightDepth:0
diameter:0
=====================
=====================
3

Process finished with exit code 0