LeetCode——Maxium Depth of Binary Tree

# 104
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.

这一题的目的是求出二叉树中的最大深度。显然用递归的方式是可以实现这一题的。无论是DFS还是BFS都是可以的。

/*** 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 {int max_depth = 0;
public:int maxDepth(TreeNode* root) {if(!root)return 0;return max_Depth(root,1);}int max_Depth(TreeNode* root,int now_depth) {if(!root)return 0;if(now_depth > max_depth) {max_depth = now_depth;}max_Depth(root -> left,now_depth + 1);max_Depth(root -> right,now_depth + 1);return max_depth;   }
};

还有一种方法，也是DFS，更加简洁。

/*** 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(root == NULL) return 0;  return 1 + max( maxDepth(root->left), maxDepth(root->right) );  }
};

除了DFS，BFS也能实现这个题目，但相对而言，BFS稍微复杂一点，需要借助到其他的结构，队列或者栈。

C++/queue

/*** 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(root == NULL) return 0;    queue<TreeNode *> Q;    Q.push(root);  int count = 1;  int depth = 0;  while(!Q.empty()){  TreeNode *tmp = Q.front();  Q.pop();  count--;  if(tmp->left){  Q.push(tmp->left);  }  if(tmp->right){  Q.push(tmp->right);  }   if(count == 0){  depth++;            count = Q.size();}  }  return depth;  }
};

C++/stack

    class Solution {  public:  int maxDepth(TreeNode *root) {  // Start typing your C/C++ solution below  // DO NOT write int main() function  if(root == NULL) return 0;  stack<TreeNode*> S;  int maxDepth = 0;  TreeNode *prev = NULL;  S.push(root);  while (!S.empty()) {  TreeNode *curr = S.top();  if (prev == NULL || prev->left == curr || prev->right == curr) {  if (curr->left)  S.push(curr->left);  else if (curr->right)  S.push(curr->right);  } else if (curr->left == prev) {  if (curr->right)  S.push(curr->right);  } else {  S.pop();  }  prev = curr;  if (S.size() > maxDepth)  maxDepth = S.size();  }  return maxDepth;  }  };

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