226-invert-binary-tree.java

/**
 * 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;
 *     }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        return helper(root);
    }

    public TreeNode helper(TreeNode node) {
        if (node == null) {
            return node;
        }
        TreeNode newLeft = node.right;
        TreeNode newRight = node.left;
        node.left = newLeft;
        node.right = newRight;
        helper(node.left);
        helper(node.right);
        return node;
    }
}

// time O(n)
// space O(n)
// using tree and dfs (preorder and recursive)