LeetCode-101. Symmetric Tree

LeetCode-101. Symmetric Tree

難度: easy

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

 

Example 1:

root = [1,2,2,3,4,4,3]
Output: true

Example 2:

root = [1,2,2,null,3,null,3]
Output: false

Constraints:

• The number of nodes in the tree is in the range [1, 1000].
• -100 <= Node.val <= 100

解題流程

1. 先看懂題目想描述甚麼情境
2. 開始想像程式運作該經過哪些流程
3. 將經過的流程所需的判斷跟細節列出
4. 實作列出的流程

先看懂題目想描述甚麼情境

開始想像程式運作該經過哪些流程

將經過的流程所需的判斷跟細節列出

實作列出的流程

typescript

分而治之

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function isSymmetric(root: TreeNode | null): boolean {
  if(!root) return true;
  function isMirror(leftNode: TreeNode | null, rightNode: TreeNode | null) {
    if(!leftNode && !rightNode) {
      return true;
    }
    if(
        leftNode?.val === rightNode?.val &&
        isMirror(leftNode.right, rightNode.left) && 
        isMirror(leftNode.left, rightNode.right)
      ) {
          return true;
        }
        return false;
      }
  return isMirror(root.left, root.right);
};

感謝閱讀，如果有錯誤或是講得不清楚的部分，再請留言指教