# 162. Find Peak Element

Difficulty: Medium

Frequency: N/A

A peak element is an element that is greater than its neighbors.

Given an input array where `num[i] ≠ num[i+1]`, find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine that `num[-1] = num[n] = -∞`.

For example, in array `[1, 2, 3, 1]`, 3 is a peak element and your function should return the index number 2.

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```class Solution {
public:
int findPeakElement(vector<int>& nums) {
if (nums.empty()) return -1;
if (nums.size() == 1) return 0;
int n = nums.size();
int lo = 0, hi = n - 1, med;
while (lo < hi) {
med = lo + (hi - lo) / 2;
if (nums[med] < nums[med + 1]) lo = med + 1;
else hi = med;
}
return lo;
}
};
```

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# 042. Trapping Rain Water

Difficulty: Hard

Frequency: N/A

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example,
Given `[0,1,0,2,1,0,1,3,2,1,2,1]`, return `6`. The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!

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```class Solution {
public:
int trap(vector<int>& height) {
if (height.size() <= 2) return 0;
int sum = 0, n = height.size();
vector<int> leftMax(n, 0);
vector<int> rightMax(n, 0);
for (int i = 1; i < n; i++) leftMax[i] = max(leftMax[i - 1], height[i - 1]);
for (int i = n - 2; i >= 0; i--) {
rightMax[i] = max(rightMax[i + 1], height[i + 1]);
int minHeight = min(leftMax[i], rightMax[i]);
if (minHeight > height[i]) sum += minHeight - height[i];
}
return sum;
}
};
```

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# 030. Substring with Concatenation of All Words

Difficulty: Hard

Frequency: N/A

You are given a string, s, and a list of words, words, that are all of the same length. Find all starting indices of substring(s) in s that is a concatenation of each word in wordsexactly once and without any intervening characters.

For example, given:
s: `"barfoothefoobarman"`
words: `["foo", "bar"]`

You should return the indices: `[0,9]`.

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class Solution {
public:
vector findSubstring(string s, vector& words) {
vector results;
if (s.empty() || words.empty()) return results;
int count = words.size(), word_length = words.size();
if (s.size() < count * word_length) return results;
int start = 0;
while (start <= s.size() – count * word_length) {
unordered_map dict;
for (int i = 0; i < words.size(); i++) dict[words[i]]++;
int head = start;
while (head 0) {
dict[tmp]–;
head += word_length;
} else break;
}
if (head == start + count * word_length) results.push_back(start);
start++;
}
return results;
}
};

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# 061. Rotate List

Difficulty: Medium

Frequency: N/A

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given `1->2->3->4->5->NULL` and k = `2`,
return `4->5->1->2->3->NULL`.

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```/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* rotateRight(ListNode* head, int k) {
if (!head) return NULL;
int len = 1;
ListNode *newHead = head, *tail = head;
while (tail->next) {
len++;
tail = tail->next;
}
tail->next = head;
if (k = k % len) {
for (int i = 0; i < len - k; i++) tail = tail->next;
}
newHead = tail->next;
tail->next = NULL;
return newHead;
}
};
```

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# 209. Minimum Size Subarray Sum

Difficulty: Medium

Frequency: N/A

Given an array of n positive integers and a positive integer s, find the minimal length of a subarray of which the sum ≥ s. If there isn’t one, return 0 instead.

For example, given the array `[2,3,1,2,4,3]` and `s = 7`,
the subarray `[4,3]` has the minimal length under the problem constraint.

click to show more practice.

More practice:If you have figured out the O(n) solution, try coding another solution of which the time complexity is O(n log n).

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```class Solution {
public:
int minSubArrayLen(int s, vector<int>& nums) {
int start = 0, sum = 0, minLen = INT_MAX;
for (int i = 0; i < nums.size(); i++) {
sum += nums[i];
while (sum >= s) {
minLen = min(minLen, i - start + 1);
sum -= nums[start++];
}
}
return (minLen == INT_MAX ? 0 : minLen);
}
};
```

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# 075. Sort Colors

Difficulty: Medium

Frequency: N/A

Given an array with n objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note:
You are not suppose to use the library’s sort function for this problem.

click to show follow up.

Follow up:
A rather straight forward solution is a two-pass algorithm using counting sort.
First, iterate the array counting number of 0’s, 1’s, and 2’s, then overwrite array with total number of 0’s, then 1’s and followed by 2’s.

Could you come up with an one-pass algorithm using only constant space?

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```class Solution {
public:
void sortColors(vector<int>& nums) {
int sec = 0;
for (int i = 0; i < nums.size(); i++) {
if (nums[i] != 0) {
for (int j = i + 1; j < nums.size(); j++) {
if (nums[j] == 0) {
nums[j] = nums[i];
nums[i] = 0;
sec = i + 1;
}
}
}
}
for (int i = sec; i < nums.size(); i++) {
if (nums[i] == 2) {
for (int j = i + 1; j < nums.size(); j++) {
if (nums[j] == 1) {
nums[j] = nums[i];
nums[i] = 1;
}
}
}
}
}
};
```

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# 259. (Locked)3Sum Smaller

Difficulty: Medium

Frequency: N/A

Given an array of n integers nums and a target, find the number of index triplets `i, j, k` with `0 <= i < j < k < n` that satisfy the condition `nums[i] + nums[j] + nums[k] < target`.

For example, given nums = `[-2, 0, 1, 3]`, and target = 2.

Return 2. Because there are two triplets which sums are less than 2:

```[-2, 0, 1]
[-2, 0, 3]
```

Follow up:
Could you solve it in O(n^2) runtime?

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```class Solution {
public:
int threeSumSmaller(vector<int>& nums, int target) {
if (nums.size() < 3) return 0;
int cnt = 0;
sort(nums.begin(), nums.end());
for (int f = 0; f < nums.size() - 2; f++) {
int s = f + 1, t = nums.size() - 1;
while (s < t) {
if (nums[f] + nums[s] + nums[t] < target) {
cnt += (t - s);
s++;
} else t--;
}
}
return cnt;
}
};
```

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