LeetCode link

first thought

  • The same as problem 1
  • How to save the node list is need to consider carefully.
  • The same root value may contains different sub nodes, so the return of the recursion must be a list too.
  • Iterate the leftNodes and rightNodes, create different roots to set them.

solution

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class Solution {
    public List<TreeNode> generateTrees(int n) {
        if (n == 0) {
            return new ArrayList<>();
        }
        int[] numbers = new int[n];
        for (int i = 0; i < n; i++) {
            numbers[i] = i + 1;
        }
        return this.dfs(numbers, 0, n - 1);
    }
    private List<TreeNode> dfs(int[] numbers, int start, int end) {
        List<TreeNode> list = new ArrayList<>();
        if (start > end) {
            list.add(null);
            return list;
        }
        for (int i = start; i <= end; i++) {
            List<TreeNode> leftNodes = this.dfs(numbers, start, i - 1);
            List<TreeNode> rightNodes = this.dfs(numbers, i + 1, end);
            for (int l = 0; l < leftNodes.size(); l++) {
                for (int r = 0; r < rightNodes.size(); r++) {
                    TreeNode node = new TreeNode(numbers[i]);
                    node.left = leftNodes.get(l);
                    node.right = rightNodes.get(r);
                    list.add(node);
                }
            }
        }
        return list;
    }
}