Java 递归实现二叉树的前序、中序、后序遍历

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;
	}
}

口诀：根 左 右

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

    private void preOrder(TreeNode root, List list) {
		if (root == null) {
			return;
		}
		list.add(root.val);
		preOrder(root.left, list);
		preOrder(root.right, list);
	}
}

口诀：左 根 右

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

口诀：左 右 根

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

    void postorder(TreeNode root, List list) {
        if (root == null) {
            return;
        }
        postorder(root.left, list);
        postorder(root.right, list);
        list.add(root.val);            
    }
}

⭐核心：注意何时遍历根节点，特别注意三种遍历方法的list.add(root.val);