LeetCode--96. Unique Binary Search Trees 95. Unique Binary Search Trees II

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96. Unique Binary Search Trees
求第n个卡特兰数

class Solution {public int numTrees(int n) {int[] dp=new int[n+1];dp[0]=1;dp[1]=1;for(int i=2;i<=n;i++){for(int root=1;root<=i;root++){dp[i]+=dp[root-1]*dp[i-root];}}return dp[n];}
}

95. Unique Binary Search Trees II

class Solution {public List<TreeNode> generateTrees(int n) {if(n==0)return new LinkedList<TreeNode>();return buildTree(1,n);}public static LinkedList<TreeNode> buildTree(int min,int max){LinkedList<TreeNode> res=new LinkedList<>();if(min>max){res.add(null);return res;}for(int i=min;i<=max;i++){LinkedList<TreeNode> leftSubTrees=buildTree(min,i-1);LinkedList<TreeNode> rightSubTrees=buildTree(i+1,max);if(leftSubTrees.size()==0 && rightSubTrees.size()==0){TreeNode root=new TreeNode(i);res.add(root);}else if(leftSubTrees.size()==0){for(TreeNode node:rightSubTrees){TreeNode root=new TreeNode(i);root.right=node;res.add(root);}}else if(rightSubTrees.size()==0){for(TreeNode node:rightSubTrees){TreeNode root=new TreeNode(i);root.left=node;res.add(root);}}else{for(TreeNode leftNode:leftSubTrees){for(TreeNode rightNode:rightSubTrees){TreeNode root=new TreeNode(i);root.left=leftNode;root.right=rightNode;res.add(root);}}}}return res; }}

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