Author: xiaoxumeng

Description:

https://leetcode.com/problems/median-of-two-sorted-arrays/#/description

Boundary & Algorithm:

Should separate odd and even.

Use recursion, so boundary includes:

• destination pos = 0: return directly
• destination pos = 1 : return min{A[0], B[0]}

Code:

class Solution {
public:
double findMedianSortedArrays(vector<int>& nums1, vector<int>& nums2) {
int m = nums1.size();
int n = nums2.size();
int total = m + n;
if (total & 0x1)
return find_kth(nums1, m, nums2, n, total / 2 + 1);
else
return (find_kth(nums1, m, nums2, n, total / 2)
+ find_kth(nums1, m, nums2, n, total / 2 + 1)) / 2;
}
private:
static double find_kth(vector<int> A, int m, vector<int> B, int n, int k) {
//always assume that m is equal or smaller than n
if (m > n) return find_kth(B, n, A, m, k);
if (m == 0) return B[k – 1];
if (k == 1) return min(A[0], B[0]);
//divide k into two parts
int pa = min(k / 2, m), pb = k – pa;
if (A[pa – 1] < B[pb – 1])
{
A.erase(A.begin(), A.begin() + pa);
return find_kth(A, m – pa, B, n, k – pa);
}
else if (A[pa – 1] > B[pb – 1])
{
B.erase(B.begin(), B.begin() + pb);
return find_kth(A, m, B, n – pb, k – pb);
}
else
return A[pa – 1];
}
};

Description:

https://leetcode.com/problems/longest-palindromic-substring/#/description

Boundary:

No

Algorithm:

1. DP (Easy to understand, but slow. Time complexity O(n^2) )
2. Manacher (fast, time complexity O(n)) http://www.cnblogs.com/grandyang/p/4475985.html

Code:

DP:

class Solution {
public:
string longestPalindrome(string s) {
int dp[s.size()][s.size()];
int left = 0,right = 0, len = 0;

for (int i = 0; i < s.size();i++)
{
for (int j = 0; j < i; j++)
{
dp[j][i] = s[i] == s[j] && (i-j < 2 || dp[j + 1][i – 1] == 1);
if(dp[j][i] && len < i – j + 1)
{
left = j;
right = i;
len = i – j + 1;
}
}
dp[i][i] = 1;
}
return s.substr(left, right – left + 1);
}
};

 

Manacher:

class Solution {
public:
// Transform S into T.
// For example, S = “abba”, T = “^#a#b#b#a#$”.
// ^ and $ signs are sentinels appended to each end to avoid bounds checking
string preProcess(string s) {
int n = s.length();
if (n == 0) return “^$”;
string ret = “^”;
for (int i = 0; i < n; i++) ret += “#” + s.substr(i, 1);
ret += “#$”;
return ret;
}
string longestPalindrome(string s) {
string T = preProcess(s);
const int n = T.length();
// 以T[i] 为中心，向左/右扩张的长度，不包含T[i] 自己，
// 因此P[i] 是源字符串中回文串的长度
vector<int> P = vector<int>(n,-1);
int C = 0, R = 0;
for (int i = 1; i < n – 1; i++) {
int i_mirror = 2 * C – i; // equals to i’ = C – (i-C)
P[i] = (R > i) ? min(R – i, P[i_mirror]) : 0;
// Attempt to expand palindrome centered at i
while (T[i + 1 + P[i]] == T[i – 1 – P[i]])
P[i]++;
// If palindrome centered at i expand past R,
// adjust center based on expanded palindrome.
if (i + P[i] > R) {
C = i;
R = i + P[i];
}
}
// Find the maximum element in P.
int max_len = 0;
int center_index = 0;
for (int i = 1; i < n – 1; i++) {
if (P[i] > max_len) {
max_len = P[i];
center_index = i;
}
}
return s.substr((center_index – 1 – max_len) / 2, max_len);
}
};

 

Tips:

1. Get a substring of a string: s.substr(start, LengthOfString);

Description:

https://leetcode.com/problems/longest-substring-without-repeating-characters/#/description

Boundary:

when s.size() ==0

Algorithm:

Initially, I just thought about discard the previous longest string and calculate the new string.

However, we cannot discard the whole previous string, the elements may be useful in the following string, such as “dvdf” ‘s result is 3 rather than 2.

Code:

class Solution {
public:
int lengthOfLongestSubstring(string s) {
if (!s.size()) return 0;
string cur = “”;
string pre = “”;
for (int k = 0; k < s.size();k++)
{
while (cur.find(s[k]) < cur.npos)
{
if (cur.size() >= pre.size())
pre = cur;
cur.erase(cur.begin());
}
cur += s[k];
}
if (cur.size() < pre.size())
return pre.size();
else
return cur.size();
}
};

Tip:

Beats 65.54%. The time complexity is O(n).

Description:

https://leetcode.com/problems/add-two-numbers/#/description

Boundary:

When one is NULL while the other is not

Algorithm:

Initially, I want to implement list2int and int2lis, however, I find that when the number gets overflow, it will get the wrong result for list2int.

Therefore, the problem should be finished as done for full adder.

When (l1 != NULL && l2 != NULL)

sum = l1 -> val+ l2 -> val + carry;

when (one is NULL and the other not)

sum = (l1 or l2) -> val + carry;

finally, if (carry), add carry to the end of list.

Code:

class Solution {
public:
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
int carry = 0;
ListNode* tmp;
ListNode *start = NULL;
ListNode *node = start;
int n1, n2, sum;
while (l1 != NULL || l2 != NULL)
{
if (l1 == NULL)
{
n1 = 0;
}
else
{
n1 = l1->val;
l1 = l1->next;
}
if (l2 == NULL)
{
n2 = 0;
}
else
{
n2 = l2->val;
l2 = l2->next;
}
sum = n1 + n2 + carry;
carry = sum / 10;

if (start == NULL)
{
start = new ListNode(sum % 10);
node = start;
}
else
{
tmp = new ListNode(sum % 10);
node->next = tmp;
node = tmp;
}
}
if (carry)
{
tmp = new ListNode(carry);
node->next = tmp;
}
return start;
}
};

Tip:

My code is identical to the example code of the fastest. However, the fastest is 26ms while mine is 55ms! Why???

Description:

https://leetcode.com/problems/reshape-the-matrix/#/description

Boundary:

r * c == r’ * c’?

Algorithm:

Store the data in one dim vector and evaluate the new r*c matrix

Code:

class Solution {
public:
vector<vector<int>> matrixReshape(vector<vector<int>>& nums, int r, int c) {
int m = nums.size();
int n = nums[0].size();

if (r * c != m * n)
return nums;

vector<int> nums_all;
vector<vector<int>> nums_new = vector<vector<int>>(r, vector<int>(c, -1));
for (int k = 0; k < m; k++)
nums_all.insert(nums_all.end(), nums[k].begin(), nums[k].end());
int count = 0;
for (int i = 0; i < r; i++)
for (int j = 0; j < c; j++)
nums_new[i][j] = nums_all[count++];
return nums_new;
}
};

Time:

 

Description:

https://leetcode.com/problems/keyboard-row/#/description

Boundary:

NO

Algorithm:

Not difficult. Use:

str.find() < str.nops

Code:

class Solution {
public:
vector<string> findWords(vector<string>& words) {
vector<string> result;
for (int k = 0; k < words.size(); k++)
{
if (line1_Big.find(words[k][0]) < line1_Big.size() || line1_Sml.find(words[k][0]) < line1_Sml.size())
{
if (ifExist(words[k], line1_Big, line1_Sml))
result.push_back(words[k]);
}
else if (line2_Big.find(words[k][0]) < line2_Big.size() || line2_Sml.find(words[k][0]) < line2_Sml.size())
{
if (ifExist(words[k], line2_Big, line2_Sml))
result.push_back(words[k]);
}
else
{
if (ifExist(words[k], line3_Big, line3_Sml))
result.push_back(words[k]);
}

}
return result;
}
private:
string line1_Big = “QWERTYUIOP”;
string line1_Sml = “qwertyuiop”;

string line2_Big = “ASDFGHJKL”;
string line2_Sml = “asdfghjkl”;

string line3_Big = “ZXCVBNM”;
string line3_Sml = “zxcvbnm”;
bool ifExist(string word, string big, string small)
{
for (int i = 1; i < word.size(); i++)
{
if (big.find(word[i]) > big.size() && small.find(word[i]) > small.size())
return false;
}
return true;
}
};

Description:

https://leetcode.com/submissions/detail/105718837/

Boundary:

No boundary needs to be considered.

Algorithm:

Use a for loop to read the char in the string one by one, record the start and end of each word.

If meets a ‘ ‘, swap poistion and store in an new string.

Code:

class Solution {
public:
string reverseWords(string s) {
string s_new = s;
int counter = 0;
int start = 0;
for (int k = 0; k < s.size();k++)
{
if (s[k] == ‘ ‘)
{
for (int i = start; i < counter; i++)
{
char s1 = s[counter – 1 – (i – start)];
s_new[i] = s1;
}

start = counter + 1;
}
else if (k == s.size() – 1)
{
for (int i = start; i <= counter; i++)
{
char s1 = s[counter – (i – start)];
s_new[i] = s1;
}
}
counter++;
}
return s_new;
}
};