二叉树先序遍历的两种思路

二叉树先序遍历的两种思路

遍历思路

• 遍历二叉树
• 首先判断一个节点应该做什么
• 然后遍历左子树 遍历右子树

/**
 * Definition for a binary tree node.
 * 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;
 *     }
 * }
 */
class Solution {
    List<Integer> res = new LinkedList<>();
    public List<Integer> preorderTraversal(TreeNode root) {

        traverse(root);
        return res;
    }

    void traverse(TreeNode root){
        if(root == null){
            return;
        }

        // 先序遍历
        res.add(root.val);
        traverse(root.left);
        traverse(root.right);
    }
}

分解思路

• 二叉树的先序遍历结果 = 根节点+ 左子树的先序遍历结果 + 右子树的先序遍历结果

/**
 * Definition for a binary tree node.
 * 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;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
            List<Integer> res = new LinkedList<>();

            if(root == null){
                return res;
            }


            // 检查一个节点要做的事情
            res.add(root.val);

            // 利用函数定义  后面接着左子树的先序遍历结果
            res.addAll(preorderTraversal(root.left));

            res.addAll(preorderTraversal(root.right));
            return res;
    }
}