leetcode:105.前シーケンスと中シーケンスを巡るシーケンス構造ツリー(java)

package LeetCode;

import java.util.ArrayList;
import java.util.List;

/*
105.                
                    。
  :
              。
  ，  
     preorder = [3,9,20,15,7]
     inorder = [9,3,15,20,7]
        ：
    3
   / \
  9  20
    /  \
   15   7
 */
public class BuildTree {

    public TreeNode buildTree(int[] preorder, int[] inorder) {

        return dfs(0, preorder.length - 1, 0, inorder.length - 1, preorder, inorder);
    }

    public TreeNode dfs(int ps, int pe, int is, int ie, int[] preorder, int[] inorder) {
        if (ps > pe) return null;
        TreeNode r = new TreeNode(preorder[ps]);
        int mid = -1;
        for (int i = 0; i <= ie; i++) {
            if (inorder[i] == preorder[ps]) {
                mid = i;
                break;
            }
        }
        if (mid == -1) return null;
        //      
        //    [ps][ps+1...ps+c][ps+c+1...pe]
        //    [is...mid-1][mid][mid+1...ie]
        int c = mid - is;
        r.left = dfs(ps + 1, ps + c, is, mid - 1, preorder, inorder);
        r.right = dfs(ps + c + 1, pe, mid + 1, ie, preorder, inorder);
        return r;
    }
}