代码随想录day19

235.二叉搜索树的最近公共祖先

//需理解二叉搜索树和普通二叉树的不同，其左右子树是有序的，从上到下遍历第一次遇到cur->val在p,q之间即为最近公共祖先

    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root == nullptr) return nullptr;
        if(root->val > p->val && root->val > q->val){
            return lowestCommonAncestor(root->left, p, q);
        }else if (root->val < p->val && root->val < q->val){
            return lowestCommonAncestor(root->right, p, q);
        }else{
            return root;
        }
        
    }

701.二叉搜索树中的插入操作

    void traverse(TreeNode* root, int val){
        if(root == nullptr) {
            return;
        }
        if(root->left == nullptr && root->right == nullptr){
            if(root->val > val){
                root->left = new TreeNode(val);
            }else{
                root->right = new TreeNode(val);
            }
            return;
        }else if( root->left == nullptr && root->right != nullptr){
            if(root->val > val){
                root->left = new TreeNode(val);
                return;
            }else{
                return traverse(root->right, val);
            }
            
        }else if (root->left != nullptr && root->right == nullptr){
            if(root->val < val){
                root->right = new TreeNode(val);
                return;
            }
            return traverse(root->left, val);
        }else{
            if(root->val > val){
                return traverse(root->left, val);
            }else{
                return traverse(root->right, val);
            }
        }
    }
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        if(root == nullptr) {
            root = new TreeNode(val);
            return root;
        }else{
            traverse(root, val);
            return root;
        }
        
    }

//简答写法，便于理解

    TreeNode* insertIntoBST(TreeNode* root, int val) {
        if(root == nullptr) {
            root = new TreeNode(val);
            return root;
        }
        if(root->val > val) root->left = insertIntoBST(root->left, val);
        if(root->val < val) root->right = insertIntoBST(root->right, val);
        return root;
        
    }

450.删除二叉搜索树中的节点，需二刷

//注意搜索树和搜索边的区别及用法，还有删除节点时左子树的位置变化

    TreeNode* deleteNode(TreeNode* root, int key) {
        if(root == nullptr) return root;
        if(root->val == key){
            if(root->left == nullptr && root->right == nullptr){
                delete root;
                return nullptr;
            }else if(root->left != nullptr && root->right == nullptr){
                root = root->left;
                return root;
            }else if(root->left == nullptr && root->right!= nullptr){
                root = root->right;
                return root;
            }else{
                TreeNode* cur = root->right;
                while(cur->left){
                    cur = cur->left;
                }
                cur->left = root->left;
                root = root->right;
                return root;
            }
        }
        if(root->val > key) root->left = deleteNode(root->left, key);
        if(root->val < key) root->right = deleteNode(root->right, key);
        return root;
    }