leetcode Maximum Depth of Binary Tree

返回树的深度。

递归：

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    int maxDepth(TreeNode *root) 

    {

        if (!root) return 0;

        int l = maxDepth(root -> left);

        int r = maxDepth(root -> right);

        return max(l + 1, r + 1);

    }

};

 非递归的解法：

public int maxDepth(TreeNode root) {

    if(root == null)

        return 0;

    

    int depth = 0;

    LinkedList<TreeNode> queue = new LinkedList<TreeNode>();

    queue.add(root);

    int curNum = 1; //num of nodes left in current level

    int nextNum = 0; //num of nodes in next level

    while(!queue.isEmpty()){

        TreeNode n = queue.poll();

        curNum--;

        if(n.left!=null){

            queue.add(n.left);

            nextNum++;

        }

        if(n.right!=null){

            queue.add(n.right);

            nextNum++;

        }

        if(curNum == 0){

            curNum = nextNum;

            nextNum = 0;

            depth++;

        }

    }

    return depth;

}

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