一、144. 二叉树的前序遍历

题目连接：https://leetcode.cn/problems/binary-tree-preorder-traversal/
思路一、递归方法
1、定义递归终止条件 if (root == null) return;
2、递归方法体：前序遍历：先中间root.val 添加到list，在左边、在右边

/**
 * Definition for a binary tree node.
 * public 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 {
    private void preorderTraversal(TreeNode root, List<Integer> list) {
        if (root == null) return;
        list.add(root.val);
        preorderTraversal(root.left, list);
        preorderTraversal(root.right, list);

    }
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        preorderTraversal(root, result);
        return result;
    }
}

思路二、使用迭代法,使用stack,先放入right,再放入left,在弹出left,依次循环
第一种写法：

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

第二种写法

/**
 * Definition for a binary tree node.
 * public 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 {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()){
            if (cur != null) {
                result.add(cur.val);
                stack.push(cur);
                cur = cur.left;
            } else {
                TreeNode treeNode = stack.pop();
                cur = treeNode.right;
            }
        }
        return result;
    }
}

二、94. 二叉树的中序遍历

题目连接：https://leetcode.cn/problems/binary-tree-inorder-traversal/
思路一、递归法，中序遍历，左中右

/**
 * Definition for a binary tree node.
 * public 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 {
    private void inorderTraversal(TreeNode root, List<Integer> list) {
        if (root == null) return;
        inorderTraversal(root.left, list);
        list.add(root.val);
        inorderTraversal(root.right, list);
    }
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        inorderTraversal(root, result);
        return result;
    }
}

思路二、迭代法,一路向走，遇到left是空的，看这个节点的右节点，右节点是空的话看栈顶的元素

/**
 * Definition for a binary tree node.
 * public 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 {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()){
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                TreeNode treeNode = stack.pop();
                result.add(treeNode.val);
                cur = treeNode.right;
            }
        }
        return result;
    }
}

三、145. 二叉树的后序遍历

题目连接：https://leetcode.cn/problems/binary-tree-postorder-traversal/
思路一、递归法,左右中

/**
 * Definition for a binary tree node.
 * public 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 {
    private void postorderTraversal(TreeNode root, List<Integer> list) {
        if (root == null) return;
        postorderTraversal(root.left, list);
        postorderTraversal(root.right, list);
        list.add(root.val);
    }
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        postorderTraversal(root, result);
        return result;
    }
}

思路二迭代法
方法一、使用前序方式 中右左遍历，最后倒序，就得到左右中

/**
 * Definition for a binary tree node.
 * public 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 {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root != null) {
            Stack<TreeNode> stack = new Stack<>();
            stack.push(root);
            while (!stack.isEmpty()) {
                TreeNode treeNode = stack.pop();
                result.add(treeNode.val);
                if (treeNode.left != null) stack.push(treeNode.left);
                if (treeNode.right != null) stack.push(treeNode.right);
            }
        }
        int left = 0;
        int right = result.size() - 1;
        while (left < right) {
            int temp = result.get(left);
            result.set(left, result.get(right));
            result.set(right, temp);
            left++;
            right--;
        }
        return result;
    }
}

方法二、访问到中间节点的时候，看它的右节点是否为空，如果为空则直接弹出。
如果不为空并且没有遍历过右节点，则遍历右节点，遍历过右节点后，最后遍历中节点

/**
 * Definition for a binary tree node.
 * public 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 {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new  Stack<>();
        TreeNode cur = root;
        TreeNode pre = null;
        while (cur != null || !stack.isEmpty()){
            if (cur != null){
                stack.push(cur);
                cur = cur.left;
            } else {
                TreeNode treeNode = stack.peek();
                //当右节点是空或者右节点访问过了，中间的节点可以弹出了
                if (treeNode.right == null || treeNode.right == pre) {
                    stack.pop();
                    result.add(treeNode.val);
                    pre = treeNode;
                    cur = null;
                } else {
                    cur = treeNode.right;
                }
            }
        }
        return result;
    }
}