代码随想录训练营Day13

144.二叉树的前序遍历 思路：这几道题都是用递归代码是比较简单的

class Solution {
    List<Integer> result = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        preorder(root);
        return result;
    }

    private void preorder(TreeNode root) {
         if (root == null) {
            return;
        }
        result.add(root.val);
        preorder(root.left);
        preorder(root.right);

    }
}

迭代法：

/**
 * 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) {
        
        Deque<TreeNode> stack = new ArrayDeque<>();
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }

        }
        return result;
    }
}

145.二叉树的后序遍历

94.二叉树的中序遍历

中序遍历和先序遍历的区别在哪？ 中序遍历是先到达的根节点，但是处理是要走到最左孩子才处理，所以跟先序遍历是不一样的

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()){
           if (cur != null){
               stack.push(cur);
               cur = cur.left;
           }else{
               cur = stack.pop();
               result.add(cur.val);
               cur = cur.right;
           }
        }
        return result;
    }
}

102.二叉树的层序遍历 leetcode.com/problems/bi…

class Solution {
    List<List<Integer>> result = new ArrayList<>();   
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) return result;
        Queue<TreeNode> que = new LinkedList<>();
        que.offer(root);

        while (!que.isEmpty()) {
            List<Integer> itemList = new ArrayList<>();
            int len = que.size();
            // 弹出节点，把节点的值加入list
            // 把弹出节点的左右孩子加入queue
            while (len > 0) {
                TreeNode tmpNode = que.poll();
                itemList.add(tmpNode.val);
                if (tmpNode.left != null) que.offer(tmpNode.left);
                if (tmpNode.right != null) que.offer(tmpNode.right);
                len--;
            }
            result.add(itemList);
        }
        return result;
    }
}