Minimum Depth of Binary Tree

Given abinary tree, find its minimum depth.

 

The minimumdepth is the number of nodes along the shortest path from the root node down tothe nearest leaf node.

 

想来也简单。以先序遍历算法为例，设置一个最小深度变量，在每个叶子节点判断当前深度是否小于最小深度，是则更新最小深度变量。

或者用深度优先遍历，当遍历到叶子节点时，当前深度即为最小深度。

 

解法一

先序遍历的递归算法。

/** * 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:    void recurTree(TreeNode* root,int &depth) {        if(root->left){            root->left->val=root->val+1;            recurTree(root->left,depth);        }        if(root->right){            root->right->val=root->val+1;            recurTree(root->right,depth);        }        if(!root->left&&!root->right){            if(root->val<depth){                depth=root->val;            }        }            }    int minDepth(TreeNode* root) {        if(!root)            return 0;        root->val=1;        int depth=2000;        recurTree(root,depth);        return depth;    }};

解法二

数组实现的层序遍历的非递归算法

/** * 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 minDepth(TreeNode* root) {        if(!root)            return 0;        TreeNode* list[2000];        int front=-1,rear=-1;        int level=0,last=0;        list[++rear]=root;        while(front<rear){            TreeNode *temp;            temp=list[++front];            if(temp->left)                list[++rear]=temp->left;            if(temp->right)                list[++rear]=temp->right;            if((!temp->left)&&(!temp->right)){                return level+1;            }            if(front==last){                last=rear;                level++;            }        }        return level;    }};

看了讨论之后，觉得自己的代码不够简洁高效。又重新写了两个改进后的代码

更简洁的dfs解法

/** * 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 minDepth(TreeNode* root) {        if(!root)            return 0;        if(root->left&&root->right)            return min(minDepth(root->left),minDepth(root->right))+1;        return max(minDepth(root->left),minDepth(root->right))+1;    }};

用queue实现的层序遍历非递归算法

/** * 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 minDepth(TreeNode* root) {        if(!root)            return 0;        queue<TreeNode*> q;        int level=0;        q.push(root);        while(!q.empty()){            int vol=q.size();            level++;            for(int i=0;i<vol;i++){                TreeNode *node;                node=q.front();                if(!node->left&&!node->right)                    return level;                if(node->left)                    q.push(node->left);                if(node->right)                    q.push(node->right);                q.pop();            }        }        return level;    }};