二叉树实现（根据层级数组生成二叉树）

public class TreeNode {
          
   
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {
          
   }
      TreeNode(int val) {
          
    this.val = val; }
      TreeNode(int val, TreeNode left, TreeNode right) {
          
   
          this.val = val;
          this.left = left;
          this.right = right;
      }

    /**
     *
     * @param arrs 层级遍历数组生成二叉树
     * @return
     */
    public static TreeNode cjdefin(Object [] arrs){
          
   
        return cjdefin(arrs, 0);
    }

    /**
     *
     * @param arrs 层级遍历数组生成树
     * @param start 父节点开始位置
     * @return
     */
      public static TreeNode cjdefin(Object [] arrs,Integer start){
          
   
          if (start > arrs.length - 1 || arrs[start] == null ){
          
   
              return null;
          }

          //声明父节点
          TreeNode treeNode = new TreeNode(Integer.parseInt(arrs[start] + ""));
          treeNode.left = cjdefin(arrs,(start + 1) * 2 -1);
          treeNode.right = cjdefin(arrs,(start + 1) * 2);

          return treeNode;
      }
  }

测试

Object [] arrs = {
          
   3,9,20,null,null,15,7};
//根据数组得到二叉树，具体如下图
TreeNode cjdefin = TreeNode.cjdefin(arrs);