654. 最大二叉树

/**
 * 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:
    TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
        if(nums.size() == 0)    return nullptr;
        int index = 0;
        for(int i = 1; i < nums.size(); i++){
            if(nums[i] > nums[index])   index = i;
        }
        TreeNode* root = new TreeNode(nums[index]);
        if(nums.size() == 1)    return root;
        vector<int> left(nums.begin(),nums.begin()+index);
        vector<int> right(nums.begin()+index+1,nums.end());
        root->left = constructMaximumBinaryTree(left);
        root->right = constructMaximumBinaryTree(right);
        return root;
    }
};

617. 合并二叉树

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 8  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 9  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
10  * };
11  */
12 class Solution {
13 public:
14     TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
15         if(root1 == nullptr)    return root2;
16         if(root2 == nullptr)    return root1;
17         root1->val += root2->val;
18         root1->left = mergeTrees(root1->left, root2->left);
19         root1->right = mergeTrees(root1->right, root2->right);
20         return root1;
21     }
22 };

700. 二叉搜索树中的搜索

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 8  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 9  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
10  * };
11  */
12 class Solution {
13 public:
14     TreeNode* searchBST(TreeNode* root, int val) {
15         if(root == nullptr) return nullptr;
16         if(root->val == val)    return root;
17         if(root->val > val) return searchBST(root->left,val);
18         else    return searchBST(root->right,val);
19     }
20 };

98. 验证二叉搜索树

中序遍历，如果是严格递增则是二叉搜索树，否则不是

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 8  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 9  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
10  * };
11  */
12 class Solution {
13 public:
14     bool isValidBST(TreeNode* root) {
15         vector<int> vec;
16         ldr(root,vec);
17         for(int i = 1; i < vec.size(); i++){
18             if(vec[i] <= vec[i-1])  return false;
19         }
20         return true;
21     }
22 private: 
23     void ldr(TreeNode* root, vector<int>& vec){
24         if(root == nullptr) return;
25         ldr(root->left,vec);
26         vec.push_back(root->val);
27         ldr(root->right,vec);
28     }
29 };