FB面经 | Algorithm

Validate a string if it is almost palindrome (remove at most one char to make it palindrome)

Two-pointer, time O(n), space O(1)

public class Solution {
    public boolean isAlmostPalindrome(String s) {
        return isAlmostPalindrome(s, 0, s.length() - 1, true);
    }
    
    public boolean isAlmostPalindrome(String s, int left, int right, boolean canSkip) {
        while (left < right) {
            if (s.charAt(left) == s.charAt(right)) {
                ++left;
                --right;
            } else if (!canSkip) {
                return false;
            } else {
                return isAlmostPalindrome(s, left + 1, right, false)
                    || isAlmostPalindrome(s, left, right - 1, false);
            }
        }
        
        return true;
    }
}

Implement an iterator API with only next() function to get node from BT with preorder traversal.

要求尽量on-the-fly，即支持多次访问。

Stack

Leetcode 173原题。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    Stack stack;
    
    public BSTIterator(TreeNode root) {
        stack = new Stack<>();
        pushAllLeft(stack, root);
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.empty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode curr = stack.pop();
        pushAllLeft(stack, curr.right);
        return curr.val;
    }
    
    private void pushAllLeft(Stack stack, TreeNode root) {
        while (root != null) {
            stack.push(root);
            root = root.left;
        }
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */

decode ways I, 唯一和LC不同是要考虑到任何input string所以有很多error/edge case要想到

621. Task Scheduler

略

543. Diameter of Binary Tree

略

KNN

各种follow up，数据量太大怎么办，KNN service的QPS很高怎么办，input是data stream怎么办

Trie

写了几个trie的基本操作，add word，search word, search word with '?' matching any single char + output all matches。最后剩不到10分钟面试官灵机一动说，你给我做个trie compression吧，就是给一个一般的trie，怎么compress trie里面可以share的path以节省存储空间，注意他不要ternary trie，也不要对unicode的volcabulary做bit level的huffman encoding，只能在原来的trie里collapse shared paths。完全没有头绪，支支吾吾说了几个idea之后只剩3分钟，下一轮面试官已经站在门口了，他居然说，sounds good，你把你的idea写出来吧。。。瞎写几行时间就到了

283. Move Zeroes

简单题，略。

76. Minimum Window Substring

TODO

binary tree inorder traversal (Iterator)

subarray sum to K

15. 3Sum变型题，数组中的每个数字可以重复使用

在原题基础上稍作改变即可，指针j从i开始，到k结束。
也可以用combination sum的思路做，k = 3。

91. Decode Ways，input可能不规范

LintCode 819. Word Sorting

Word Sorting

25. Reverse Nodes in k-Group

34. Search for a Range

Minesweeper

Given a height and a width and number of mines, return a minesweeper
field with mines in random positions
input: 2, 3, 3
return:

---

• • • X -

---

• X - X - -

---

我用loop，每次random一个0~width*height的数，然后转化为x和y，给mine二位数组赋值

follow up question：

1. what if input: 100, 100, 9999. 雷太多怎么办
2. what if input: 10000, 10000, 10000*10000/2. 上一题我答的用雷过半就算没雷的地方，保证不过半。然后他就问如果刚好一半怎么办，如何检测conflict

88. Merge Sorted Array

略

138. Copy List with Random Pointer

TODO

658. Find K Closest Elements

TODO

269. Alien Dictionary变型题

知道字典排序 判定字符串是不是按顺序排 就是反过来