95. Unique Binary Search Trees II（js）

95. Unique Binary Search Trees II

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3
题意：给定一个数字，求出所有组成的二叉搜索树
代码如下：

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number} n
 * @return {TreeNode[]}
 */
var generateTrees = function(n) {
    if(n<1){
        return [];
    }
    return backtrack(1,n);
};
var backtrack=function(start,end){
    var res=[];
    if(start>end){
        res.push(null);
        return res;
    }
    for(var i=start;i<=end;i++){
        var leftNode=backtrack(start,i-1);
        var rightNode=backtrack(i+1,end);
        for(var l=0;l<leftNode.length;l++){
            for(var r=0;r<rightNode.length;r++){
                var root=new TreeNode(i);
                root.left=leftNode[l];
                root.right=rightNode[r];
                res.push(root);
                
            }
        }
    }
    return res;
}