輸入一棵二叉樹，求該樹的深度。從根結點到葉結點依次經過的結點（含根、葉結點）形成樹的一條路徑，最長路徑的長度為樹的深度

解題思路

遞迴寫法，比較簡單

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

非遞迴寫法

depth 是當前結點所在的層數，count 是已經遍歷過的結點數，nextCount 是下層的結點總數，當 count == nextCount 的時候，代表本層的節點已經遍歷完畢

import java.util.LinkedList;
import java.util.Queue;
public class Solution {
    
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int depth = 0, count = 0, nextCount = 1;
        while(queue.size() != 0) {
            TreeNode top = queue.poll();
            count++;
            if(top.left != null) {
                queue.add(top.left);
            }
            if(top.right != null) {
                queue.add(top.right);
            }
            if(count == nextCount) {
                nextCount = queue.size();
                count = 0;
                depth++;
            }
        }
        return depth;
    }
}