[Leetcode][第257题][JAVA][二叉树的所有路径][BFS][DFS]

刺骨的言语ヽ痛彻心扉 2022-12-03 15:47 141阅读 0赞

#### 【问题描述】\[简单\] ####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2RhZG9uZ3d1ZGk_size_16_color_FFFFFF_t_70_pic_center]

#### 【解答思路】 ####

###### 1. DFS ######

![在这里插入图片描述][2020090410181097.png_pic_center]

**时间复杂度：O(N^2) 空间复杂度：O(N^2)**  
![在这里插入图片描述][202009041018219.png_pic_center]

class Solution { 
        public List<String> binaryTreePaths(TreeNode root) { 
            List<String> paths = new ArrayList<String>();
            constructPaths(root, "", paths);
            return paths;
        }
    
        public void constructPaths(TreeNode root, String path, List<String> paths) { 
            if (root != null) { 
                StringBuffer pathSB = new StringBuffer(path);
                pathSB.append(Integer.toString(root.val));
                if (root.left == null && root.right == null) {   // 当前节点是叶子节点
                    paths.add(pathSB.toString());  // 把路径加入到答案中
                } else { 
                    pathSB.append("->");  // 当前节点不是叶子节点，继续递归遍历
                    constructPaths(root.left, pathSB.toString(), paths);
                    constructPaths(root.right, pathSB.toString(), paths);
                }
            }
        }
    }

时间复杂度更高 ，使用String+“ ”拼接字符串

class Solution { 
    
        //这行代码可不敢写这里，leetcode提交会出问题的。。。。!!!
        //private static List<String> res = new ArrayList<String>();
    
        public List<String> binaryTreePaths(TreeNode root) { 
    
            List<String> res = new ArrayList<String>();
            if(root == null)return res;
            BL(root,res,"");
    
            return res;
        }
    
        public static void BL(TreeNode node,List<String> res,String str){ 
    
            if(node.left != null) BL(node.left, res, str + node.val + "->");
    
            if(node.right != null) BL(node.right, res, str + node.val + "->");
            
            if(node.left == null && node.right == null) res.add(str + node.val);
            
        }
    }
    
    
    链接：https://leetcode-cn.com/problems/binary-tree-paths/solution/chang-gui-cao-zuo-50di-gui-by-gu-xiong-007/

###### 2. BFS ######

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2RhZG9uZ3d1ZGk_size_16_color_FFFFFF_t_70_pic_center 1]

**时间复杂度：O(N2) 空间复杂度：O(N2)**  
![在这里插入图片描述][20200904102930357.png_pic_center]

class Solution { 
        public List<String> binaryTreePaths(TreeNode root) { 
            List<String> paths = new ArrayList<String>();
            if (root == null) { 
                return paths;
            }
            Queue<TreeNode> nodeQueue = new LinkedList<TreeNode>();
            Queue<String> pathQueue = new LinkedList<String>();
    
            nodeQueue.offer(root);
            pathQueue.offer(Integer.toString(root.val));
    
            while (!nodeQueue.isEmpty()) { 
                TreeNode node = nodeQueue.poll(); 
                String path = pathQueue.poll();
    
                if (node.left == null && node.right == null) { 
                    paths.add(path);
                } else { 
                    if (node.left != null) { 
                        nodeQueue.offer(node.left);
                        pathQueue.offer(new StringBuffer(path).append("->").append(node.left.val).toString());
                    }
    
                    if (node.right != null) { 
                        nodeQueue.offer(node.right);
                        pathQueue.offer(new StringBuffer(path).append("->").append(node.right.val).toString());
                    }
                }
            }
            return paths;
        }
    }

#### 【总结】 ####

###### 1. String StringBuffer StringBuilder ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2RhZG9uZ3d1ZGk_size_16_color_FFFFFF_t_70_pic_center 2] ######

###### 2.二叉树遍历 ######

*  前序遍历 先输出当前结点的数据，再依次遍历输出左结点和右结点
 *  中序遍历 先遍历输出左结点，再输出当前结点的数据，再遍历输出右结点
 *  后续遍历 先遍历输出左结点，再遍历输出右结点，最后输出当前结点的数据

###### 3.复制粘贴时候要小心 修改相应代码 脑子模拟代码 快速排查错误 多从自己写的代码 切忌浮躁 ######

转载链接：https://leetcode-cn.com/problems/binary-tree-paths/solution/er-cha-shu-de-suo-you-lu-jing-by-leetcode-solution/  
参考链接：https://leetcode-cn.com/problems/binary-tree-paths/solution/chang-gui-cao-zuo-50di-gui-by-gu-xiong-007/

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