目录

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

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递归看这篇文章：三道题套路解决递归问题 | lyl's blog

         前序遍历的和后序遍历的迭代方法代码思路相同；因为栈为先进后出，所以前序遍历的入栈顺序为中右左；后序遍历为左右中，反过来则为中右左，入栈顺序为中左右；

        中序遍历的思路则大不相同，具体见代码；

144. 二叉树的前序遍历

 代码：

// 递归方法
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        preorder(root, list);
        return list;
    }
    public void preorder(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        list.add(root.val);
        preorder(root.left, list);
        preorder(root.right, list);
    }
}

// 迭代方法
// 前序遍历：中左右；入栈顺序：中右左
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 node = stack.pop();
            list.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return list;
    }
}

94. 二叉树的中序遍历

 代码：

// 递归遍历
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        inorder(root, list);
        return list;
    }
    public void inorder(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }        
        inorder(root.left, list);
        list.add(root.val);
        inorder(root.right, list);
    }
}

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

145. 二叉树的后序遍历

 代码：

// 递归方法
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        postorder(root, list);
        return list;
    }
    public void postorder(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        
        postorder(root.left, list);
        postorder(root.right, list);
        list.add(root.val);
    }
}

// 迭代方法
// 后序遍历：左右中；反过来为中右左,入栈顺序：中左右
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            list.add(node.val);
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        Collections.reverse(list);
        return list;
    }
}