层序遍历——队列实现

给你二叉树的根节点 root ，返回其节点值的层序遍历。（即逐层地，从左到右访问所有节点）

分析

借助队列存储的方式实现。队列这个数据结构是先入先出的。

具体步骤：
1、将根节点入队
2、出队首节点，将队首节点的左右非空孩子入队
3、重复2操作直到队列为空

注意：Java的队列由linkedList实现的，这个需要注意一下。

Java完整代码

/**
 * 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<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();  
        if (root == null) {
            return res;
        }
        LinkedList<TreeNode> queue = new LinkedList<>(); // Java的队列由linkedList实现的
        queue.add(root);
        while (!queue.isEmpty()) {
            List<Integer> list = new ArrayList<>();
            int current_queue_size = queue.size();
            for (int i = 0; i < current_queue_size; i++) {
                TreeNode top = queue.getFirst();
                list.add(top.val);
                if (top.left != null) {
                    queue.add(top.left);
                }
                if (top.right != null) {
                    queue.add(top.right);
                }
                queue.removeFirst();
            }
            res.add(list);
        }
        return  res;
    }
    
}

先序遍历——中左右

给你二叉树的根节点 root ，返回它节点值的前序遍历。

分析

前序遍历是先访问根节点再左孩子然后右孩子。

递归实现

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        preorder(root, res);
        return res;
    }

    public void preorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        res.add(root.val);
        preorder(root.left, res);
        preorder(root.right, res);
    }
}

非递归实现——栈实现

借助栈数据结构

 * 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) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        TreeNode tmp;

        while (p != null || !stack.isEmpty()) {
            // 一路向左
            if (p != null) {
                result.add(p.val);
                stack.push(p);
                p = p.left;
            } else {
                tmp = stack.pop();
                p = tmp.right;
            }
        }
        return result;

    }
}

中序遍历——左中右

递归实现

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        inorder(root, res);
        return res;
    }

    public void inorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        inorder(root.left, res);
        res.add(root.val);
        inorder(root.right, res);
    }
}

非递归实现——栈实现

/**
 * 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<>();
        if (root == null) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        TreeNode tmp;

        while (p != null || !stack.isEmpty()) {
            // 一路向左
            if (p != null) {
                stack.push(p);
                p = p.left;
            } else {
                tmp = stack.pop();
                result.add(tmp.val);
                p = tmp.right;
            }
        }
        return result;
    }
}

后续遍历——左右中

递归实现

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        postorder(root, res);
        return res;
    }

    public void postorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        postorder(root.left, res);
        postorder(root.right, res);
        res.add(root.val);
    }
}

非递归实现——栈加标志指针实现

/**
 * 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) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        TreeNode tmp;
        TreeNode review = null;

        while (p != null || !stack.isEmpty()) {
            // 一路向左
            if (p != null) {
                stack.push(p);
                p = p.left;
            } else {
                tmp = stack.peek();
                if (tmp.right == null || review == tmp.right) {
                    result.add(tmp.val);
                    review = stack.pop();
                } else {
                    p = tmp.right;
                }
            }
        }
        return result;
    }
}