树-相关的基础算法代码总结

主要罗列了下代码，看到代码后应该会清晰一些。

1.先序遍历（非递归）

	//先序遍历
	public void PreOrder(TreeNode root){
		if(null == root){
			return;
		}
		
		Stack<TreeNode> stk = new Stack<TreeNode>();
		
		while(root!=null || !stk.isEmpty()){
			
			while(root != null){
				System.out.println(root.val);
				stk.push(root);
				root = root.left;
			}
			
			if(!stk.isEmpty()){//栈不为空
				root = stk.pop();
				root = root.right;
			}
			
		}
		
		
	}

2.中序遍历（非递归）

	//中序遍历
	public void InOrder(TreeNode root){
		if(root == null){
			return;
		}
		
		Stack<TreeNode> stk = new Stack<TreeNode>();
		
		while(root != null || !stk.isEmpty()){
			
			while(root != null){
				stk.push(root);
				root = root.left;
			}
			
			if(!stk.isEmpty()){
				root = stk.pop();
				System.out.println(root.val);
				root = root.right;
			}
		}
		
	}

3.后序遍历（非递归）

	//后续遍历
	public void PostOrder(TreeNode root){
		
		if(null == root){
			return;
		}
		
		Stack<TreeNode> stk = new Stack<TreeNode>();
		Stack<TreeNode> stk1 = new Stack<TreeNode>();
		
		stk.push(root);
		
		while(!stk.isEmpty()){
			root = stk.pop();
			stk1.push(root);
			
			if(root.left != null){
				stk.push(root.left);
			}
			
			if(root.right != null){
				stk.push(root.right);
			}
		}//end while
		
		//输出
		while(!stk1.isEmpty()){
			System.out.println(stk1.pop().val);
		}
		
		
	}

4.层次遍历输出

	//层次遍历--队列
	public ArrayList<Integer> PrintFromTopToBottom(TreeNode root){
		
		ArrayList<Integer> resultList = new ArrayList<Integer>();//作为结果返回
		Queue<TreeNode> queue = new LinkedList<TreeNode>();
		TreeNode current;
		
		if(root == null){
			return resultList;
		}else{
			
			queue.offer(root);
			resultList.add(root.val);

			while(!queue.isEmpty()){
				current = queue.poll();
				if(current.left != null){
					queue.offer(current.left);
					resultList.add(current.left.val);
				}
				
				if(current.right != null){
					queue.offer(current.right);
					resultList.add(current.right.val);
				}
				
			}//end while--结束

		}
		
		return resultList;
	}

5.求树的深度

	//树的深度
	public int TreeDepth(TreeNode root){
		
		if(null == root){
			return 0;
		}
		
		Queue<TreeNode> queue = new LinkedList<TreeNode>();
		int cur,last;
		int level = 0;
		TreeNode current;
		
		queue.offer(root);
		while(!queue.isEmpty()){//队列不为空
			
			cur = 0;
			last = queue.size();
			
			while(cur<last){
				current = queue.poll();
				if(current.left != null){
					queue.offer(current.left);
				}
				
				if(current.right != null){
					queue.offer(current.right);
				}
				cur++;//这个必须得操作  不然是死循环
			}//end while
			
			level++;
		}
		
		return level;
	}

6.树的之字形打印

	//树的之字形打印
	public void ZigZagPrint(TreeNode root){
		
		if(null == root){
			return;
		}
		Stack<TreeNode> stk = new Stack<TreeNode>();
		Stack<TreeNode> stk1 = new Stack<TreeNode>();
		
		stk.push(root);
		while(!stk.isEmpty() || !stk1.isEmpty()){
			
			while(!stk.isEmpty()){
				TreeNode node = stk.pop();
				System.out.println(node.val);
				if(node.left != null){
					stk1.push(node.left);
				}
				if(node.right!=null){
					stk1.push(node.right);
				}
				
			}//end while
			
			while(!stk1.isEmpty()){
				TreeNode node = stk1.pop();
				System.out.println(node.val);
				if(node.right != null){
					stk.push(node.right);
				}
				
				if(node.left != null){
					stk.push(node.left);
				}
				
				
			}//end while
		}//end outer while
	}