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• 原题连接：543. 二叉树的直径

1- 思路

深搜——dfs

递归三部曲

• 1- 递归参数和返回值
  • 返回 public int depth(TreeNode root)
• 2- 终止条件
  • 如果遇到 null，则返回 0
• 3- 递归表达式
  • return Math.max(left,right)+1

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2- 实现

⭐543. 二叉树的直径——题解思路

class Solution {
    int res = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        depth(root);
        return res;
    }

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

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3- ACM 实现

public class diameterOfBinaryTree {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode() {}
        TreeNode(int val) { this.val = val; }
        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

    public static TreeNode build(String str){
        if(str == null || str.length()==0){
            return null;
        }
        String input = str.replace("[","");
        input = input.replace("]","");
        String[] parts = input.split(",");

        Integer[] nums = new Integer[parts.length];
        for(int i = 0 ; i < parts.length;i++){
            if(!parts[i].equals("null")){
                nums[i] = Integer.parseInt(parts[i]);
            }else{
                nums[i] = null;
            }
        }
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode root = new TreeNode(nums[0]);
        queue.offer(root);
        int index = 1;
        while(!queue.isEmpty()&& index<parts.length){
            TreeNode node = queue.poll();
            if(index<nums.length && nums[index]!=null){
                node.left = new TreeNode(nums[index]);
                queue.offer(node.left);
            }
            index++;
            if(index<nums.length && nums[index]!=null){
                node.right = new TreeNode(nums[index]);
                queue.offer(node.right);
            }
            index++;
        }
        return root;
    }

    static int res = 0;
    public static int diameterOfBinaryTree(TreeNode root) {
        depth(root);
        return res;
    }

    public static int depth(TreeNode root){
        if(root==null){
            return 0;
        }
        int left = depth(root.left);
        int right = depth(root.right);
        res = Math.max(res,left+right);
        return Math.max(left,right)+1;
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine();
        TreeNode root = build(input);
        System.out.println("结果是"+diameterOfBinaryTree(root));

    }
}