leetcode 104 Maximum Depth of Binary Tree二叉树求深度

Maximum Depth of Binary Tree
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Question Solution

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.

我的解决方案：

/**
 * Definition for a binary tree node.
 * 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(NULL == root)
            return 0;

        int depth_l = maxDepth(root->left);
        int depth_r = maxDepth(root->right);

        return depth_l > depth_r  ? depth_l + 1:depth_r + 1;

    }
};

一行代码的解法：

int maxDepth(TreeNode *root)
{
    return root == NULL ? 0 : max(maxDepth(root -> left), maxDepth(root -> right)) + 1;
}

不用递归的解法：Breadth-first-search

int maxDepth(TreeNode *root)
{
    if(root == NULL)
        return 0;

    int res = 0;
    queue<TreeNode *> q;
    q.push(root);
    while(!q.empty())
    {
        ++ res;
        for(int i = 0, n = q.size(); i < n; ++ i)
        {
            TreeNode *p = q.front();
            q.pop();

            if(p -> left != NULL)
                q.push(p -> left);
            if(p -> right != NULL)
                q.push(p -> right);
        }
    }

    return res;
}

不用递归的解法2

int maxDepth(TreeNode *root)
{
    if (root == NULL) return 0;
    stack<TreeNode *> gray;
    stack<int> depth;
    int out = 0;

    gray.push(root);
    depth.push(1);
    while (!gray.empty()) {
        TreeNode *tmp = gray.top();
        int num = depth.top();
        gray.pop();
        depth.pop();
        if (tmp->left == NULL && tmp->right == NULL) {
            out = num > out ? num : out;
        }
        else {
            if (tmp->left != NULL) {
                gray.push(tmp->left);
                depth.push(num + 1);
            }
            if (tmp->right != NULL) {
                gray.push(tmp->right);
                depth.push(num + 1);
            }
        }
    }
    return out;
}

python 的解决方案：

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return an integer
    def maxDepth(self, root):

        def maxDepthHelper(root):
            if not root: return 0
            return max(1+maxDepthHelper(root.left), 1+maxDepthHelper(root.right))

        return maxDepthHelper(root)