【树】 leetcode_222 _ 完全二叉树的节点个数

后序递归

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int countNodes(TreeNode* root) {
        int result = get_depth(root);
        return result;
    }
    int get_depth(TreeNode *cur){
        if (cur== nullptr) return 0;
        int le = get_depth(cur->left);
        int right = get_depth(cur->right);
        int result = le + right+1;
        return result;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int countNodes(TreeNode* root) {
        if (root == nullptr){return 0;}
        return countNodes(root->left);+countNodes(root->right);+1;

    }
};

层序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int countNodes(TreeNode* root) {
        TreeNode *cur = root;
        if (cur == nullptr) return 0;
        int num = 0;
        queue<TreeNode *> q_node;
        q_node.push(cur);
        while(!q_node.empty()){
            int n = q_node.size();
            for(int i = 0; i<n; i++){
                cur = q_node.front();
                q_node.pop();
                num++;
                if(cur->left) q_node.push(cur->left);
                if(cur->right) q_node.push(cur->right);
            }
        }
        return num;

    }
};