技術標籤：演算法

什麼是棧？

1、棧是一種先進先出的線性結構

2、只能從棧頂插入或者刪除。

jdk中棧的資料結構

提供了入棧的push操作，和出棧的pop操作。有資料結構有：

1、執行緒安全的Stack

2、LinkedList

應用：

二叉樹的中序遍歷

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 {
      LinkedList<TreeNode> stack = new LinkedList<>();
      List<Integer> returnList = new ArrayList<>();

       while (!stack.isEmpty() || root != null) {

             if (root != null) {
                 stack.push(root);
                 root = root.left;
                 continue;
             }

           root = stack.pop();
           returnList.add(root.val);
           root = root.right ;

       }
        return returnList;
}
 

二叉樹的前序遍歷

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
       
        List<Integer> returnList = new ArrayList<>();
        LinkedList<TreeNode> stack = new LinkedList<>();

        while (!stack.isEmpty() || root != null) {

            if (root != null) {
                returnList.add(root.val);
                stack.push(root);
                root = root.left;
                continue;
            }

            root = stack.pop().right;
        }

        return returnList;
    }
}
 

二叉樹的後序遍歷

public List<Integer> postorderTraversal(TreeNode root) {
           LinkedList<TreeNode> stack = new LinkedList<>();
        List<Integer> returnList = new ArrayList<>();
        TreeNode prev = null;
        while (!stack.isEmpty() || root != null) {


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

            root = stack.pop();
            if (root.right == null || root.right == prev) {
                returnList.add(root.val);
                prev = root;
                root = null;
            } else {
                stack.push(root);
                root = root.right;
            }

        }
        return returnList;
    }