[leetcode]minimum-depth-of-binary-tree

题目描述

Given a binary tree, find its minimum depth.The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

解法

• 递归方式：很容易理解，直接上代码

class Solution {
public:
    int run(TreeNode *root) {
        if(!root) return 0;
        if(!root -> left) return 1 + run(root -> right);
        if(!root -> right) return 1 + run(root -> left);
        return 1 + min(run(root -> left), run(root -> right));
    }
};

• 非递归方式：基于队列对二叉树做层序遍历，用一个整型变量记录层数。层序遍历遇到的第一个叶节点即是“最浅”的叶节点，此时返回层数

class Solution {
public:
    int run(TreeNode *root) {
        if(!root) return 0;
        
        queue<TreeNode*> q;
        q.push(root);
        int res = 0; // 记录层数
        while(!q.empty()) {
            ++res;
            for(int i = q.size(); i > 0; --i) { // 遍历第res层，把res+1层的节点添加到队列
                auto p = q.front();
                q.pop();
                if(!p -> left && !p -> right) // 遇到第一个叶节点即返回层数
                    return res;
                if(p -> left)
                    q.push(p -> left);
                if(p -> right)
                    q.push(p -> right);
            }
        }
        
        return res;
    }
};