Number of Ways to Reorder Array to Get Same BST LeetCode solution in Java

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class Solution {
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    long[][] pascal;
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     private static final long mod = 1000000007;
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    long one=1;
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    public int numOfWays(int[] nums) {
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        ArrayList list = new ArrayList<>();
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        for (int n : nums) {
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            list.add(n);
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        }
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        formPascal(nums);
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        return (int)dfs(list)-1;
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    }
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    public long dfs(ArrayList list){
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        if(list.size()<=2) return 1;
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        ArrayList left=new ArrayList<>();
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        ArrayList right=new ArrayList<>();
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        int root=list.get(0);
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        for(int n:list){
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            if(nroot)
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                right.add(n);
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        }
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        return ((pascal[left.size()+right.size()][left.size()])*(dfs(left)%mod))%mod *(dfs(right)%mod);
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    }
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    private void formPascal(int[] nums){
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        pascal=new long[nums.length+1][];
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        for(int i=0;i<=nums.length;i++){
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            pascal[i]=new long[i+1];
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           Arrays.fill(pascal[i],one);
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            for(int j=1;j