题解 | #二叉树的前序遍历#

/*
 * 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
     */
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        List<Integer> list = new ArrayList<>();
        while(root != null){
            if(root.left != null){
                TreeNode pre = root.left;
                while(pre.right != null && pre.right != root){
                    pre = pre.right;
                }
                if(pre.right != root){
                    list.add(root.val);
                    pre.right = root;
                    root = root.left;
                }else{
                    pre.right = null;
                    root = root.right;
                }
            }else{
            list.add(root.val);
            root = root.right;
        }
      }
       int[] nums = new int[list.size()];
        int i = 0;
        for(int l : list){
            nums[i++] = l;
        }
        return nums;
    }
}