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专栏：Java数据结构秘籍
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目录
一、非递归实现遍历二叉树
1.1. 二叉树的前序遍历
1.2. 二叉树的中序遍历
1.3. 二叉树的后序遍历
一、非递归实现遍历二叉树
1.1. 二叉树的前序遍历
我们这里要使用栈来进行实现。我们反向思考一下为什么不使用队列？如下图，前序遍历肯定是先将根结点放进去，如果是队列，根结点先进先出，然后怎么去遍历右子树呢，就无法打印的顺序了。
我们定义一个引用cur，只要cur不为null，就打印值并将该元素放入栈中。当遍历到4时，左子树为空，返回结点4并弹出，再去遍历4的右结点，然后返回结点2并弹出，让cur等于结点2的右子树并遍历。只要1的左子树没有遍历完，1就不弹出。
public class Solution {public void preorderTraversal(TreeNode root){if(root == null){return;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while(cur != null){stack.push(cur);System.out.print(cur.val+" ");cur = cur.left;}}
}代码写到这里就会出现问题，原因是：当遍历到结点4的时候，4的左子树为空，就无法进入while循环。然后把4弹出去，让cur=top，问题又来了，如果结点4左边要是不为空，又得放入栈中，也需要走while循环。
我们会发现当cur走到某个结点时，如果为空，但栈不为空，此时就可以巧妙地在while外面再加一层while循环。
while (cur != null || !stack.isEmpty()) {while (cur != null) {stack.push(cur);System.out.print(cur.val + " ");cur = cur.left;}cur = stack.pop();cur = cur.right;
}完整代码实现：
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;class TreeNode{int val;TreeNode left;TreeNode right;public TreeNode() {}public TreeNode(int val) {this.val = val;}public TreeNode(int val, TreeNode left, TreeNode right) {this.val = val;this.left = left;this.right = right;}
}public class Solution {public List<Integer> preorderTraversal(TreeNode root){List<Integer> tree = new ArrayList<>();if(root == null){return tree;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while (cur != null || !stack.isEmpty()) {while (cur != null) {tree.add(cur.val);stack.push(cur);cur = cur.left;}cur = stack.pop();cur = cur.right;}return tree;}
}import java.util.ArrayList;
import java.util.List;public class Test {public static void main(String[] args) {List<Integer> result = new ArrayList<>();Solution solution = new Solution();TreeNode root = new TreeNode(1,new TreeNode(2),new TreeNode(3));root.left.left = new TreeNode(4);root.left.right = new TreeNode(5);root.left.right.left = new TreeNode(6);root.left.right.right = new TreeNode(7);root.right.right = new TreeNode(8);root.right.right.left = new TreeNode(9);result = solution.preorderTraversal(root);System.out.println(result);}
}1.2. 二叉树的中序遍历
与前序遍历的思路相同，只是打印的时机不一样。中序遍历要在弹出的元素之后直接打印。
完整代码实现：
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;class TreeNode{int val;TreeNode left;TreeNode right;public TreeNode() {}public TreeNode(int val) {this.val = val;}public TreeNode(int val, TreeNode left, TreeNode right) {this.val = val;this.left = left;this.right = right;}
}public class Solution {public List<Integer> inorderTraversal(TreeNode root){List<Integer> tree = new ArrayList<>();if(root == null){return tree;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while (cur != null || !stack.isEmpty()) {while (cur != null) {tree.add(cur.val);stack.push(cur);cur = cur.left;}cur = stack.pop();tree.add(cur.val);cur = cur.right;}return tree;}
}
import java.util.ArrayList;
import java.util.List;public class Test {public static void main(String[] args) {List<Integer> result = new ArrayList<>();Solution solution = new Solution();TreeNode root = new TreeNode(1,new TreeNode(2),new TreeNode(3));root.left.left = new TreeNode(4);root.left.right = new TreeNode(5);root.left.right.left = new TreeNode(6);root.left.right.right = new TreeNode(7);root.right.right = new TreeNode(8);root.right.right.left = new TreeNode(9);result = solution.inorderTraversal(root);System.out.println(result);}
}
1.3. 二叉树的后序遍历
后序遍历不能按照我们上面前序与中序的方法来做。如果结点下面还有孩子结点，如果把4弹出之后，就无法获取它的右侧，所以只能获取不能弹出。当右子树为空，才能弹出，再进行打印。
public class Solution {public void postorderTraversal(TreeNode root){if(root == null){return;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;TreeNode top = null;while(cur != null || !stack.isEmpty()) {while(cur != null){stack.push(cur);cur = cur.left;}top = stack.peek();if(top.right == null){stack.pop();System.out.print(top.val+" ");}else{cur = top.right;}}}
}public class Test {public static void main(String[] args) {Solution solution = new Solution();TreeNode root = new TreeNode(1,new TreeNode(2),new TreeNode(3));root.left.left = new TreeNode(4);root.left.right = new TreeNode(5);root.left.right.right = new TreeNode(7);root.right.right = new TreeNode(8);root.right.right.left = new TreeNode(9);solution.postorderTraversal(root);}
}
但这样写，会存在问题：当遍历到结点5的右结点7时，会陷入死循环。那我们怎么知道这个结点被打印过？我们再定义引用prev，让prev来记录被弹出的结点。
while(cur != null || !stack.isEmpty()) {while(cur != null){stack.push(cur);cur = cur.left;}top = stack.peek();if(top.right == null || top.right == prev){stack.pop();System.out.print(top.val+" ");prev = top;}else{cur = top.right;}完整代码实现：
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;class TreeNode{int val;TreeNode left;TreeNode right;public TreeNode() {}public TreeNode(int val) {this.val = val;}public TreeNode(int val, TreeNode left, TreeNode right) {this.val = val;this.left = left;this.right = right;}
}public class Solution {public List<Integer> postorderTraversal(TreeNode root){List<Integer> tree = new ArrayList<>();if(root == null){return tree;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;TreeNode top = null;TreeNode prev = null;while(cur != null || !stack.isEmpty()) {while(cur != null){stack.push(cur);cur = cur.left;}top = stack.peek();if(top.right == null || top.right == prev){tree.add(top.val);stack.pop();prev = top;}else{cur = top.right;}}return tree;}
}import java.util.ArrayList;
import java.util.List;public class Test {public static void main(String[] args) {List<Integer> tree = new ArrayList<>();Solution solution = new Solution();TreeNode root = new TreeNode(1,new TreeNode(2),new TreeNode(3));root.left.left = new TreeNode(4);root.left.right = new TreeNode(5);root.left.right.left = new TreeNode(6);root.left.right.right = new TreeNode(7);root.right.right = new TreeNode(8);root.right.right.left = new TreeNode(9);tree = solution.postorderTraversal(root);System.out.println(tree);}
}