二叉树遍历

二叉树的先序、中序、后序遍历 递归、迭代实现

递归：
先序

public void PreOrder(TreeNode root){
	if(root == null) return;
	System.out.println(root.val);
	if(root.left != null ){
		PreOrder(root.left);
	}
	if(root.right != null){
		PreOrder(root.right);
	}
}

中序：

public void InOrder(TreeNode root){
	if(root == null)  return;
	if(root.left != null){
		InOrder(root.left);
	}
	System.out.println(root.val);
	if(root.right != null){
		InOrder(root.right);
	}
} 

后序：

public void BeOrder(TreeNode root){
	if(root == null) return;
	if(root.left != null){
		BeOrder(root.left);
	}
	if(root.right != null){
 		BeOrder(root.right);
 	}
 	System.out.println(root.val);
}

迭代实现：

先序：

public void PreOrder(TreeNode root){
	//List<Integer> list = new ArrayList<>();
	Stack<TreeNode> stack = new Stack<>();
	while(root != null || !stack.isEmpty()){
		while(root != null){
			//list.add(root.val);
			System.out.println(root.val);
			stack.push(root);
			root = root.left;
		}
		root = stack.pop();
		root = root.right;
	}
}

中序：

public void InOrder(TreeNode root){
	Stack<TreeNode> stack = new Stack<>();
	while( root != null || !stack.isEmpty()){
		while(root != null){
			stack.push(root);
			root = root.left;
		}
		root = stack.pop();
		System.out.println(root.val);
		root = root.right;
	}
}

后序：

public void BeOrder(TreeNode root){
	Stack<TreeNode> stack = new Stack<>();
	TreeNode pre= null;
	while(root != null || !stack.isEmpty()){
		while(root != null){
			stack.push(root);
			root = root.left;
		}
		root = stack.peek();
		if(root.left == null || root.right == p){
			System.out.println(root.val);
			stack.pop();
			p = root;
			root = null;
		}else{
			root = root.right;
		}
	}
}