二叉树构建

public static class TreeNode{
    int val;
    TreeNode left;
    TreeNode right;
    public TreeNode(int val) {
        this.val = val;
    }
    public TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}
public static TreeNode construct(int[] arr){
    List<TreeNode> list = new ArrayList<>();
    TreeNode root = null;
    int n = arr.length;
    for(int i=0;i<n;i++){
        TreeNode node = new TreeNode(arr[i]);
        list.add(node);
        if(i==0)
            root = node; // 构建根节点
    }
    for(int i=0;i*2+1<n;i++){
        TreeNode node = list.get(i);
        if(node!=null){
            node.left = list.get(i*2+1);
            if(i*2+2<n){
                node.right = list.get(i*2+2);
            }
        }
    }
    return root;
}