LC.P1261[在受污染的二叉树中查找元素]

方法一：DFS+哈希表

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/**
 * 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 FindElements {

    Set<Integer> set = new HashSet<>();

    public FindElements(TreeNode root) {
        root.val = 0;
        dfs(root);
    }
    
    public boolean find(int target) {
        return set.contains(target);
    }

    private void dfs(TreeNode root) {
        if (root == null) return;
        int x = root.val;
        set.add(x);
        if (root.left != null) root.left.val = 2 * x + 1;
        if (root.right != null) root.right.val = 2 * x + 2;
        dfs(root.left);
        dfs(root.right);
    }
}

/**
 * Your FindElements object will be instantiated and called as such:
 * FindElements obj = new FindElements(root);
 * boolean param_1 = obj.find(target);
 */
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$