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Formatted question description: https://leetcode.ca/all/1382.html
1382. Balance a Binary Search Tree (Medium)
Given a binary search tree, return a balanced binary search tree with the same node values.
A binary search tree is balanced if and only if the depth of the two subtrees of every node never differ by more than 1.
If there is more than one answer, return any of them.
Example 1:
Input: root = [1,null,2,null,3,null,4,null,null] Output: [2,1,3,null,null,null,4] Explanation: This is not the only correct answer, [3,1,4,null,2,null,null] is also correct.
Constraints:
- The number of nodes in the tree is between
1
and10^4
. - The tree nodes will have distinct values between
1
and10^5
.
Related Topics:
Binary Search Tree
Solution 1.
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/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public TreeNode balanceBST(TreeNode root) { if (root == null || root.left == null && root.right == null) return root; List<TreeNode> inorderTraversal = inorderTraversal(root); return balanceBST(inorderTraversal); } public List<TreeNode> inorderTraversal(TreeNode root) { List<TreeNode> inorderTraversal = new ArrayList<TreeNode>(); Stack<TreeNode> stack = new Stack<TreeNode>(); TreeNode node = root; while (!stack.isEmpty() || node != null) { while (node != null) { stack.push(node); node = node.left; } TreeNode visitNode = stack.pop(); inorderTraversal.add(visitNode); node = visitNode.right; } return inorderTraversal; } public TreeNode balanceBST(List<TreeNode> inorderTraversal) { int size = inorderTraversal.size(); if (size == 1) { TreeNode root = new TreeNode(inorderTraversal.get(0).val); return root; } else if (size == 2) { TreeNode root = new TreeNode(inorderTraversal.get(1).val); TreeNode leftChild = new TreeNode(inorderTraversal.get(0).val); root.left = leftChild; return root; } else if (size == 3) { TreeNode root = new TreeNode(inorderTraversal.get(1).val); TreeNode leftChild = new TreeNode(inorderTraversal.get(0).val); TreeNode rightChild = new TreeNode(inorderTraversal.get(2).val); root.left = leftChild; root.right = rightChild; return root; } else { int rootIndex = (size - 1) / 2; TreeNode root = new TreeNode(inorderTraversal.get(rootIndex).val); List<TreeNode> leftInorderTraversal = new ArrayList<TreeNode>(); List<TreeNode> rightInorderTraversal = new ArrayList<TreeNode>(); for (int i = 0; i < rootIndex; i++) leftInorderTraversal.add(inorderTraversal.get(i)); for (int i = rootIndex + 1; i < size; i++) rightInorderTraversal.add(inorderTraversal.get(i)); root.left = balanceBST(leftInorderTraversal); root.right = balanceBST(rightInorderTraversal); return root; } } } ############ /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { private List<Integer> vals; public TreeNode balanceBST(TreeNode root) { vals = new ArrayList<>(); dfs(root); return build(0, vals.size() - 1); } private void dfs(TreeNode root) { if (root == null) { return; } dfs(root.left); vals.add(root.val); dfs(root.right); } private TreeNode build(int i, int j) { if (i > j) { return null; } int mid = (i + j) >> 1; TreeNode root = new TreeNode(vals.get(mid)); root.left = build(i, mid - 1); root.right = build(mid + 1, j); return root; } }
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// OJ: https://leetcode.com/problems/balance-a-binary-search-tree/ // Time: O(H) // Space: O(N) class Solution { vector<int> v; void inorder(TreeNode *root) { if (!root) return; inorder(root->left); v.push_back(root->val); inorder(root->right); } TreeNode *build(int start, int end) { if (start >= end) return NULL; int mid = (start + end) / 2; auto node = new TreeNode(v[mid]); node->left = build(start, mid); node->right = build(mid + 1, end); return node; } public: TreeNode* balanceBST(TreeNode* root) { inorder(root); return build(0, v.size()); } };
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# Definition for a binary tree node. # class TreeNode: # def __init__(self, val=0, left=None, right=None): # self.val = val # self.left = left # self.right = right class Solution: def balanceBST(self, root: TreeNode) -> TreeNode: def dfs(root): if root is None: return dfs(root.left) vals.append(root.val) dfs(root.right) def build(i, j): if i > j: return None mid = (i + j) >> 1 root = TreeNode(vals[mid]) root.left = build(i, mid - 1) root.right = build(mid + 1, j) return root vals = [] dfs(root) return build(0, len(vals) - 1)
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/** * Definition for a binary tree node. * type TreeNode struct { * Val int * Left *TreeNode * Right *TreeNode * } */ func balanceBST(root *TreeNode) *TreeNode { var vals []int var dfs func(root *TreeNode) dfs = func(root *TreeNode) { if root == nil { return } dfs(root.Left) vals = append(vals, root.Val) dfs(root.Right) } dfs(root) var build func(i, j int) *TreeNode build = func(i, j int) *TreeNode { if i > j { return nil } mid := (i + j) >> 1 return &TreeNode{vals[mid], build(i, mid-1), build(mid+1, j)} } return build(0, len(vals)-1) }