代码随想录算法训练营第14天|二叉树的前中后序的遍历（递归和迭代）

今日内容： 

●  二叉树理论基础

●  递归遍历  

●  迭代遍历

●  统一迭代

144. 二叉树的前序遍历

先是递归写法

class Solution {
    List<Integer> res;
    public List<Integer> preorderTraversal(TreeNode root) {
        res =new ArrayList<>();
        recur(root);
        return res;
    }
    private void recur(TreeNode root) {
        if(root == null){
            return;
        }
        res.add(root.val);
        recur(root.left);
        recur(root.right);
    }
}

然后是迭代写法

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

94. 二叉树的中序遍历

先是递归写法

class Solution {
    private List<Integer> res;
    public List<Integer> inorderTraversal(TreeNode root) {
        res = new ArrayList<Integer>();
        find(root);
        return res;
    }
    private void find(TreeNode root) {
        if(root == null) {
            return;
        }
        find(root.left);
        res.add(root.val);
        find(root.right);
    }
}

然后是迭代写法 中序遍历的迭代和其他两种不一样

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> res = new ArrayList<>();
        if(root == null)return res;
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()) {
            if(cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.pop();
                res.add(cur.val);
                cur = cur.right;
            }
        }
        return res;
    }
}

145. 二叉树的后序遍历

先是递归写法

class Solution {
    List<Integer> res;
    public List<Integer> postorderTraversal(TreeNode root) {
    res =new ArrayList<>();
        recur(root);
        return res;
    }
    private void recur(TreeNode root) {
        if(root == null){
            return;
        }
        
        recur(root.left);
        recur(root.right);
        res.add(root.val);
    }
}

然后是迭代写法

中右左入栈，然后再反转

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