java_invert_binary_tree

Given the root  of a binary tree, invert the tree, and return its root.

Examples

Example 1:

Input: root = [4,2,7,1,3,6,9]
Output: [4,7,2,9,6,3,1]

Example 2:

Input: root = [2,1,3]
Output: [2,3,1]

Example 3:

Input: root = []
Output: []

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

解析

題目給定一個二元樹的 root

實作一個演算法把數中所有原本左邊的結點換成右邊的結點

已知每個結點都由左結點與右邊結點組成

所以 要反轉整個樹的左右結點等於 把root 的左右結點互換 + 原本左子樹做反轉 + 原本右子樹做反轉

程式碼

class Solution {
  /**
   * Definition for a binary tree node.
   * 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;
   *     }
   * }
   */
  public TreeNode invertTree(TreeNode root) {
    if (root == null) {
      return root;
    }
    TreeNode left = root.left;
    TreeNode right = root.right;
    root.left = invertTree(right);
    root.right = invertTree(left);
    return root;
  }
}

困難點

1. 看出每個轉換結點的遞迴關係

Solve Point

• Understand what problem would like to solve
• Analysis Complexity