代码随想录算法训练营day16 | ● 104.二叉树的最大深度 559.n叉树的最大深度 ● 111.二叉树的最小深度 ● 222.完全二叉树的节点个数

104.二叉树的最大深度

后序遍历法

class Solution {
public:
    int getdepth(TreeNode* node){
        if(node == NULL)    return 0;
        int leftdepth = getdepth(node->left);
        int rightdepth = getdepth(node->right);
        int depth = 1 + max(leftdepth, rightdepth);
        return depth;
    }

    int maxDepth(TreeNode* root) {
        return getdepth(root);
    }
};

111. 二叉树的最小深度

后序

class Solution {
public:
    int getDepth(TreeNode* node){
        if(node == NULL)    return 0;
        int leftDepth = getDepth(node->left);
        int rightDepth = getDepth(node->right);

        if(node->left == NULL && node->right != NULL){
            return 1 + rightDepth;
        }
        if(node->left != NULL && node->right == NULL){
            return 1 + leftDepth;
        }
        int result = 1 + min(leftDepth, rightDepth);
        return result;
    }

    int minDepth(TreeNode* root){
        return getDepth(root);
    }
};

222.完全二叉树的节点数量

class Solution {
public:
    int countNodes(TreeNode* root) {
        if(root == NULL) return 0;
        TreeNode* left = root->left;
        TreeNode* right = root->right;
        int leftDepth = 0, rightDepth = 0;
        while(left){
            left = left->left;
            leftDepth++;
        }
        while(right){
            right = right->right;
            rightDepth++;
        }
        if(leftDepth == rightDepth){
            return (2 << leftDepth) - 1; // <<为“位运算符”，(2<<1) 相当于2^2，所以leftDepth初始为0
        }
        return 1 + countNodes(root->left) + countNodes(root->right); 
    }
};