题目

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

解答

求二叉树的最大深度。

递归：a.若二叉树为空，深度为0;b.若二叉树不为空，深度=max(左子树深度，右子树深度)+1；迭代：用队列实现。代码（含注释）如下：

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

//迭代法
public int maxDepth(TreeNode root){
	if(root==null){
		return 0;
	}
	Queue<TreeNode> queue=new LinkedList<TreeNode>();
	queue.add(root);

	int depth=0;
	int currentLevelNodes=1;   //当前层，node的数量
	int nextLevelNodes=0;      //下一层，node的数量

	while(!queue.isEmpty()){
		TreeNode cur=queue.remove();   //从队头位置移除(先进来的的先被移除)
		currentLevelNodes--;           //减少当前层node的数量
		if(cur.left!=null){
			nextLevelNodes++;
			queue.add(cur.left);
		}
		if(cur.right!=null){
			nextLevelNodes++;
			queue.add(cur.right);
		}
		if(currentLevelNodes==0){    //说明已经遍历当前层的所有节点
			depth++;
			currentLevelNodes=nextLevelNodes;
			nextLevelNodes=0;
		}
	}
	return depth;
}

---EOF---

【LeetCode】Maximum Depth of Binary Tree