【leetcode】Maximum Depth of Binary Tree

Question ：

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.

Anwser 1 ：  DFS

/** * 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) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(root == NULL) return 0;                int l = maxDepth(root->left);        int r = maxDepth(root->right);                return l > r ? l + 1 : r + 1;    }};

More simpe ：

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;        return 1 + max( maxDepth(root->left), maxDepth(root->right) );    }};

Anwser 2 ：    BFS   in  queue

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;                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++;            // one row is end, add a depth                count = Q.size();   // next row count            }        }        return depth;    }};

Anwser 3 ： BFS   in  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;    }};

參考推薦：

Maximum Depth of Binary Tree

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