144.二叉树的前序遍历 94.二叉树的中序遍历 145.二叉树的后序遍历

144.二叉树的前序遍历 94.二叉树的中序遍历 145.二叉树的后序遍历

一、使用递归遍历二叉树

//前序遍历
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        
        traverSal(root, result);
        return result;
    }
    public void traverSal(TreeNode node, List result){
        if(node == null) return;
        
        result.add(node.val);
        traverSal(node.left, result);
        traverSal(node.right, result);
    }
}

//中序遍历
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        return travelVal(list, root);
    }

    public List<Integer> travelVal(List list, TreeNode cur){
        if (cur == null) return list;

        travelVal(list, cur.left);
        list.add(cur.val);
        travelVal(list, cur.right);
        return list;
    }
}

//后序遍历
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        travelval(root, list);
        return list;
    }
    //递归调用
    public void travelval(TreeNode cur, List list){
        if (cur == null) return;
        //后序：左右中
        travelval(cur.left, list);
        travelval(cur.right, list);
        list.add(cur.val);
    }
}

二、迭代遍历

使用栈来做

//前序
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list =new ArrayList<>();
        if (root == null) return list;

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode treeNode = stack.pop();
            list.add(treeNode.val);
            if (treeNode.right != null) stack.push(treeNode.right);
            if (treeNode.left != null) stack.push(treeNode.left);
        }
        return list;
    }
}

 //中序：左中右
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while(cur!= null || !stack.isEmpty()){
            if(cur != null){
                stack.push(cur);
                cur = cur.left;
            }else{
                cur = stack.pop();
                list.add(cur.val);
                cur = cur.right;
            }

        }
        return list;
    }
}

//后续
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.left != null){
                stack.push(node.left);
            }
            if (node.right != null){
                stack.push(node.right);
            }
        }
        Collections.reverse(result);
        return result;
    }
}

总结

构造递归的三个步骤：
1、确定递归函数的参数和返回值类型。
2、终止条件。
3、确定单层递归的逻辑