669. Trim a Binary Search Tree

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problem

solution

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/**
 * 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* trimBST(TreeNode* root, int L, int R) {
        //只是將Node的指向進行的更改，而不是實際刪除Node
        if (root == nullptr) return nullptr;
        // Node 太小，那返回他的右子樹，右子樹比他大
        if (root->val < L) return trimBST(root->right, L, R);
        if (root->val > R) return trimBST(root->left, L, R);
        // 遞迴
        root->left = trimBST(root->left, L, R);
        root->right = trimBST(root->right, L, R);
        return root;
    }
};

analysis

  • time complexity O(n)
  • space complexity O(n)