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78 lines
1.6 KiB
Markdown
Executable File
78 lines
1.6 KiB
Markdown
Executable File
# [95. Unique Binary Search Trees II](https://leetcode.com/problems/unique-binary-search-trees-ii/)
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## 题目
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Given an integer *n*, generate all structurally unique **BST's** (binary search trees) that store values 1 ... *n*.
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**Example**:
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Input: 3
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Output:
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[
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[1,null,3,2],
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[3,2,null,1],
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[3,1,null,null,2],
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[2,1,3],
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[1,null,2,null,3]
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]
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Explanation:
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The above output corresponds to the 5 unique BST's shown below:
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1 3 3 2 1
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\ / / / \ \
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3 2 1 1 3 2
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/ / \ \
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2 1 2 3
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## 题目大意
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给定一个整数 n，生成所有由 1 ... n 为节点所组成的二叉搜索树。
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## 解题思路
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- 输出 1~n 元素组成的 BST 所有解。这一题递归求解即可。外层循环遍历 1~n 所有结点，作为根结点，内层双层递归分别求出左子树和右子树。
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## 代码
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```go
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package leetcode
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/**
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* Definition for a binary tree node.
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* type TreeNode struct {
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* Val int
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* Left *TreeNode
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* Right *TreeNode
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* }
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*/
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func generateTrees(n int) []*TreeNode {
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if n == 0 {
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return []*TreeNode{}
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}
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return generateBSTree(1, n)
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}
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func generateBSTree(start, end int) []*TreeNode {
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tree := []*TreeNode{}
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if start > end {
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tree = append(tree, nil)
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return tree
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}
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for i := start; i <= end; i++ {
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left := generateBSTree(start, i-1)
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right := generateBSTree(i+1, end)
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for _, l := range left {
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for _, r := range right {
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root := &TreeNode{Val: i, Left: l, Right: r}
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tree = append(tree, root)
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}
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}
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}
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return tree
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}
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``` |