Leetcode 95. Unique Binary Search Trees II 二叉搜索树2 解题报告

1 解题思想

2 原题

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.
For example,
Given n = 3, your program should return all 5 unique BST's shown below.
1         3     3      2      1
\       /     /      / \      \
3     2     1      1   3      2
/     /       \                 \
2     1         2                 3

3 AC解

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*
* http://blog.csdn.net/chilseasai/article/details/50083201 */
public class Solution {
public List<TreeNode> helper(int start,int end){
List<TreeNode> result = new ArrayList<TreeNode>();
if( start > end){
result.add(null);
return result;
}
/**
这里就是上一篇里所说的，确定两边，然后构造树的，具体原理请参照上一篇
**/
for(int i = start; i <= end; i ++) {
List<TreeNode> leftChild = helper(start, i - 1);
List<TreeNode> rightChild = helper(i + 1, end);
for(TreeNode left : leftChild) {
for(TreeNode right : rightChild) {
TreeNode root = new TreeNode(i);
root.left = left;
root.right = right;
result.add(root);
}
}
}
return result;

}
public List<TreeNode> generateTrees(int n) {
if(n < 1)
return new ArrayList<TreeNode>();
return helper(1,n);

}
}