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;
}