题解 | #二叉树的前序遍历#

方法一：用递归

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    private List<Integer> list=new ArrayList<>();
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        list=preOrder(root);
        int[] res= new int[list.size()];
        for(int i=0;i<list.size();i++){
            res[i]=list.get(i);
        }
        return res;
    }
    
    List<Integer> preOrder(TreeNode node){
        if(node==null){
            return list;
        }
        list.add(node.val);
        preOrder(node.left);
        preOrder(node.right);
        return list;
        
    }
}

方法二：用栈

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    private List<Integer> list=new ArrayList<>();
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        if(root==null) return new int[0];
        Stack<TreeNode> stack=new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node=stack.pop();
            list.add(node.val);
            
            if(node.right!=null) stack.push(node.right);
            if(node.left!=null) stack.push(node.left);
        }
        int[] res= new int[list.size()];
        for(int i=0;i<list.size();i++){
            res[i]=list.get(i);
        }
        return res;
    }
    
}