day20 二叉樹7

235. 二叉搜索樹的最近公共祖先

遞歸法：

class Solution {
public:
    TreeNode* traversal(TreeNode* cur, TreeNode* p, TreeNode* q) {
        if (cur->val > p->val && cur->val > q->val) {
            TreeNode* left = traversal(cur->left, p, q);
            if (left != nullptr) {
                return left;
            }
        }
        if (cur->val < p->val && cur->val < q->val) {
            TreeNode* right = traversal(cur->right, p, q);
            if (right != nullptr) {
                return right;
            }
        }
        return cur;
    }

    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        return traversal(root, p, q);
    }
};

迭代法：

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        while (root) {
            if (root->val < p->val && root->val < q->val) {
                root = root->right;
            }
            else if (root->val > p->val && root->val > q->val) {
                root = root->left;
            }
            else {
                return root;
            }
        }
        return root;
    }
};

701.二叉搜索樹中的插入操作

遞歸法：

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

迭代法：

class Solution {
public:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        TreeNode *node = new TreeNode(val);
        if (root == nullptr) {
            return node;
        }
        TreeNode *cur = root;
        TreeNode *parent = root;
        while (cur) {
            parent = cur;
            if (cur->val > val) {
                cur = cur->left;
            }
            else {
                cur = cur->right;
            }
        }
        if (val < parent->val) {
            parent->left = node;
        }
        else {
            parent->right = node;
        }
        return root;
    }
};

450.刪除二叉搜索樹中的節(jié)點(diǎn)

class Solution {
public:
    TreeNode* deleteNode(TreeNode* root, int key) {
        if (root == nullptr) {
            return root;
        }
        if (root->val == key) {
            //左空，右空
            if (root->left == nullptr && root->right == nullptr) {
                delete root;
                return nullptr;
            }
            //左空，右不空
            else if (root->left == nullptr) {
                auto right = root->right;
                delete root;
                return right; 
            }
            //左不空唧垦，右空
            else if (root->right == nullptr) {
                auto left = root->left;
                delete root;
                return left;
            }
            //左不空，右不空
            else {
                TreeNode* cur = root->right;
                while (cur->left != nullptr) {
                    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;
    }
};