Binary search for a peak. Other peaks can be ignored.

Binary Search. Create a start to index mapping. Sort the intervals by start asc. For each interval i, binary search i.end among starts and return the

Tree Map. Same idea, but tree map already has the implementation ready. Tree map's keys are sorted and has the API to get ceiling/floor or lower/highe

Second largest element is smaller than or equal to max. When current number is >= max, update second max and max. Note the equal sign since when anoth

Recursive. 0, 1, 8 are absolutely strobogrammatic. 6, 9 form a pair. When given n > 2, 0 cannot be put at the outside most position. Recurrence relati

Recursive. Build the number from outside to inside. Each position can have multiple choices. A few edge cases: 1. The first digit cannot be 0, unless

Do a DFS for each character in the given matrix

Position of integer n should be n - 1 if sorted Correct form [1, 2, 3, 4, ..., #, n] If not in position swap it with nums[nums[p]-1]

Use string characters as index and build a count array. Augment the count array by storing not just counts but also the index of the first time you en

A simpler version. If a number is both divisible by 3 and 5, both "Fizz" and "Buzz" will be added to the string builder. There's no need for another s

A more verbose version by adding each case strictly.

Valid input, denominator can't be zero Convert to long to avoid overflow Divide into three parts, sign, before dot and after dot Before dot = numerato

String. First figure out how many words to fit current line. | Init a length as -1, since the first word doesn't have a space. | Start from i, add wor

recursively calculate the greateset common divisor of two numbers

recursively calculate the greateset common divisor of two numbers

Euclid's Algorithm. gcd(a, 0) = a gcd(a, b) = gcd(b, a mod b) a mod b = a - b floor(a / b)

Track current level Work level by level toward center

get the value from trie with the key @return the value of the key. otherwise, return null

Build an array of character count and the index of its first appearance

Return the getHeight of a node. Recurrence relation: getHeight(node) = 1 + max(getHeight(node.left), getHeight(node.right)) Base case: If node is null

Get the height of each child of root. If the height is smaller than max height, do nothing. If the height is larger than max height, recurse down. If

Heap. Merge. Just like how we merge 2 sorted arrays, we can do the same to N arrays. The only difference is that a min-heap is needed to find the next

Find the minimum value of BST.

Brute-force. O(mn) Time. Generate all distances of each numbers pair. Maintain a minimum as result. <p> BUD analysis. Bottleneck? Unnecessary? Duplica

Same as Word Ladder 1. Transform a word by replacing each character with a different character, one by one. If the transformed word is in given dictio

Remove invalid parentheses as we traverse through the string, character by character. Def. of an invalid parentheses is that it makes the current stri

DFS. Must avoid cycles during traversal. Add a set of visited node to achieve that. If there is a cycle, add a path. Or if there is no more node to tr

generate 0, 1 then add 10 from back to get 11, 10 same goes for 00, 01, 11, 10, add 100 to get 110, 111, 101, 100

Binary Search. Think about the definition of h index: h papers that have >= h citations. If randomly pick an index in the citations array, mid. The #

Pure bit manipulation "n &= n - 1" is used to delete the right "1" of n Stop when all 1s are deleted and n is zero

Most straight forward solution Iterate 32 times to check each digit in n

Recursive If n is 0 or 1, return n If not, return n & 1 + hammingWeightC(n >>> 1)

DFS. With node visited states and node on stack states.

Build the left subtree first. As size shrinks, we can reach the position of next node in tree. Set the node and its left child. Move head to next link

Get copy node from map

Post order traversal

Recursive, DFS Divide into left subtree and right subtree with indices range Choose mid point as the root of subtree

Backtracking

Recursive, DFS Build a helper function to pass cur result If its leaf node, just return the val Otherwise, goes to left root first then right root wit

Get relative digit from list First digit's index in list is k / factorial(n-1) Get the digit, remove that digit from list and update k Concatenate dig

DP. Similar to find two minimum values. The only difference is we don't update second min until first min is found. Otherwise sec min can be before fi

DP. An easier to understand version. First quick verify the input array. Then initialize according to the first 2 values. Then start iterating from th

Build an adjacency list and an incoming degree array of graph. Don't know if its acyclic yet. Have to check later in topological sort. @param indegre

Iterative. Must move current node pointer correctly. How? If cur is not null, push it to stack and move it to its left. If cur is null, 2 cases: 1. cu

The largest height difference of 2 leaves must be <= 1. How to find the largest height difference? By traversing all heights can be generated. Just ne

DFS. Each edge connects nodes from different sets. So if a node is added to one set, all its neighbors are added to another. Then when it comes to the

Each node has only 3 states: not visited, in one set, in the other set. So use int[] to replace hash map to run faster. 0 means not visited. 1 means o

loop detection like linked list

loop detection using Set, use more space

DP. O(mn) Time. O(mn) Space. Can optimize to 1d array. ' ' can match empty or any sequence. Recurrence Relation: If p[j-1] != ' ': | s[i-1] and p[j-1]

Two Pointers. Greedy. Backtracking. Different from regex matching since ' ' cannot shrink the preceding character. ' ' can match any sequence. So it t

Two Pointers. Find the middle node, reverse the right half list, then check each node. Use two pointers, one slow pointer s, one fast pointer f. Move

Two pointers. Ask for clarification: What characters do we have for input? Space? Case sensitive or not? First convert the string to lowercase. Then s