二叉樹的前序周遊，中序周遊，後序周遊代碼

前序周遊

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans=new LinkedList<Integer>();
        Stack<TreeNode> stack= new Stack<TreeNode>();
        if(root!=null) stack.push(root);
        while(!stack.isEmpty())
        {
            TreeNode node=stack.pop();
            ans.add(node.val);
            if(node.right!=null) stack.push(node.right);
            if(node.left!=null) stack.push(node.left);
        }
        return ans;

    }
}
           

中序周遊

public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans=new LinkedList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        while(true){

            while(root!=null)
            {
                stack.push(root);
                root=root.left;
            }
            if(stack.isEmpty()) break;

            TreeNode node=stack.pop();
            ans.add(node.val);
            root=node.right;

        }
        return ans;
    }
}
           

後序周遊

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