java将树形结构转为list

你可以使用递归的方式来将树形结构转为list。具体来说，你可以先将根节点加入list中，然后递归处理每个子节点，并将它们加入list中。实现代码如下：

public List<TreeNode> flatten(TreeNode root) {
    List<TreeNode> result = new ArrayList<>();
    flattenHelper(root, result);
    return result;
}

private void flattenHelper(TreeNode node, List<TreeNode> result) {
    result.add(node);
    for (TreeNode child : node.getChildren()) {
        flattenHelper(child, result);
    }
}

这里假设树节点的定义如下：

public class TreeNode {
    private int val;
    private List<TreeNode> children;
    
    public TreeNode(int val) {
        this.val = val;
        children = new ArrayList<>();
    }
    
    public void addChild(TreeNode child) {
        children.add(child);
    }
    
    public List<TreeNode> getChildren() {
        return children;
    }
}

如果你希望将树形结构转为list的同时保留节点的层级信息，可以使用一个helper类来记录每个节点的层级：

public class FlattenedTreeNode {
    private TreeNode node;
    private int level;
    
    public FlattenedTreeNode(TreeNode node, int level) {
        this.node = node;
        this.level = level;
    }
    
    public TreeNode getNode() {
        return node;
    }
    
    public int getLevel() {
        return level;
    }
}

public List<FlattenedTreeNode> flatten(TreeNode root) {
    List<FlattenedTreeNode> result = new ArrayList<>();
    flattenHelper(root, 0, result);
    return result;
}

private void flattenHelper(TreeNode node, int level, List<FlattenedTreeNode> result) {
    result.add(new FlattenedTreeNode(node, level));
    for (TreeNode child : node.getChildren()) {
        flattenHelper(child, level + 1, result);
    }
}