[Vssue]二叉树的迭代遍历.md

[youngyangyang04]

https://www.programmercarl.com/%E4%BA%8C%E5%8F%89%E6%A0%91%E7%9A%84%E8%BF%AD%E4%BB%A3%E9%81%8D%E5%8E%86.html

[Du1in9]

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>(); 
        Stack<TreeNode> st = new Stack<>();    
        if (root == null) return res;

        st.push(root);
        while (!st.isEmpty()) {
            TreeNode node = st.pop();     
            res.add(node.val);            
            if (node.right != null) st.push(node.right);
            if (node.left != null) st.push(node.left);
        }
        return res;
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>(); 
        Stack<TreeNode> st = new Stack<>();    
        if (root == null) return res;

        st.push(root);
        while (!st.isEmpty()) {
            TreeNode node = st.pop();     
            res.add(node.val);            
            if (node.left != null) st.push(node.left);
            if (node.right != null) st.push(node.right);
        }
        Collections.reverse(res);
        return res;
    }
}

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>(); 
        Stack<TreeNode> st = new Stack<>();    
        if (root == null) return res;

        TreeNode node = root;               
        while (node != null || !st.isEmpty()) {
            if (node != null) {
                st.push(node);        
                node = node.left;        
            } else {
                node = st.pop();      
                res.add(node.val);    
                node = node.right;     
            }
        }
        return res;
    }
}

[csu-yulin]

// 前序遍历：根左右
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        Deque<TreeNode> stk = new LinkedList<TreeNode>();
        while (root != null || !stk.isEmpty()) {
            while (root != null) {
                res.add(root.val);
                stk.push(root);
                root = root.left;
            }
            root = stk.pop();
            root = root.right;
        }
        return res;
    }
}

// 中序遍历：左根右
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        Deque<TreeNode> stk = new LinkedList<TreeNode>();
        while (root != null || !stk.isEmpty()) {
            while (root != null) {
                stk.push(root);
                root = root.left;
            }
            root = stk.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }
}

// 后序遍历：左右根
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new LinkedList<Integer>();
        Deque<TreeNode> stack = new LinkedList<TreeNode>();

        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                result.addFirst(root.val);
                root = root.right;
            }

            root = stack.pop();
            root = root.left;
        }

        return result;
    }
}