[leetCode]104.二叉树的最大深度

解法一 递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        else{
            int leftHeight = maxDepth(root.left);
            int rightHeight = maxDepth(root.right);
            return Math.max(leftHeight,rightHeight)+1;
        }
    }
}

解法二 广优先搜索

首先加入根结点，每次弹出一个结点,加入左右子节点并跟新每个结点的深度，使用一个遍历不断更新最大深度

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        Queue<Pair<TreeNode, Integer>> stack = new LinkedList<>();
        if(root != null){
            stack.add(new Pair(root, 1));
        }
        int depth = 0;
        while(!stack.isEmpty()){
            Pair<TreeNode, Integer> current = stack.poll();
            root = current.getKey();
            int currentMaxH = current.getValue();
            if(root != null){
                depth = Math.max(currentMaxH, depth);
                stack.add(new Pair(root.left,depth + 1));
                stack.add(new Pair(root.right,depth + 1));
            }
        }
        return depth;
    }
}

每次往栈中添加一层结点，遍历取出该层结点添加下一层非空结点，每层遍历完成，层级+1

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        Queue<TreeNode> stack = new LinkedList<>();
        stack.add(root);
        int level = 0;
        while(!stack.isEmpty()){
            int levelSize = stack.size();//每一层的节点数
            for(int i = 0; i < levelSize; i++){//每次弹出一层结点，添加下一层结点
                root  = stack.poll();
                if(root.left!=null){
                    stack.add(root.left);
                }
                if(root.right!=null){
                    stack.add(root.right);
                }
            }
            level++;
        }
        return level;
    }
}