數組問題要明確變量的定義，並全程保持其定義，維護循環不變量。
維護循環不變量
283 Move Zeroes
https://leetcode.com/problems/move-zeroes/
Given an array nums, write a function to move all 0’s to the end of it while maintaining the relative order of the non-zero elements.
public void moveZeroes(int[] nums) {
if (nums == null) {
return;
}
int j = 0;
for (int i=0; i<nums.length; i++) {
if (nums[i] != 0) {
nums[j++] = nums[i];
}
}
for (; j<nums.length; j++) {
nums[j] = 0;
}
}
27 Remove Element
https://leetcode.com/problems/remove-element/
Given an array nums and a value val, remove all instances of that value in-place and return the new length.
public int removeElement(int[] nums, int val) {
int j=0;
for (int i=0; i<nums.length; i++) {
if (nums[i] != val) {
nums[j] = nums[i];
j++;
}
}
return j;
}
26 Remove Duplicates from Sorted Array
https://leetcode.com/problems/remove-duplicates-from-sorted-array/
Given a sorted array nums, remove the duplicates in-place such that each element appear only once and return the new length.
public int removeDuplicates(int[] nums) {
int j=0;
for (int i=0; i<nums.length; i++) {
if (i==nums.length-1) {
nums[j] = nums[i];
j++;
return j;
}
if (nums[i] != nums[i+1]) {
nums[j] = nums[i];
j++;
}
}
return j;
}
80 Remove Duplicates from Sorted Array II
https://leetcode.com/problems/remove-duplicates-from-sorted-array-ii/
Given a sorted array nums, remove the duplicates in-place such that duplicates appeared at most twice and return the new length.
亂寫一通，居然通過了。而且Runtime: 0 ms, faster than 100.00%，Memory Usage: 37.2 MB, less than 100.00%。
public int removeDuplicates(int[] nums) {
int i=0;
int j=0;
while(i<nums.length) {
if (i+2>=nums.length) {
nums[j] = nums[i];
i++;
j++;
continue;
}
if (nums[i+2] != nums[i]) {
nums[j] = nums[i];
j++;
}
i++;
}
return j;
}
Partition思路
75 Sort Colors
https://leetcode.com/problems/sort-colors/
Given an array with n objects colored red, white or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white and blue.
public void sortColors(int[] nums) {
int j = 0;
int k = nums.length-1;
for (int i=0; i<=k; i++) {
if (nums[i] == 0) {
swap(nums, i, j++);
} else if (nums[i] == 2) {
swap(nums, i--, k--);
}
}
}
private void swap(int[] nums, int i, int j) {
int c = nums[i];
nums[i] = nums[j];
nums[j] = c;
}
88 Merge Sorted Array
https://leetcode.com/problems/merge-sorted-array/
Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.
public void merge(int[] nums1, int m, int[] nums2, int n) {
int i1 = m-1;
int i2 = n-1;
for(int i=m+n-1; i>=0; i-- ){
if (i2<0 && i1>=0) {
nums1[i] = nums1[i1];
i1--;
continue;
} else if (i1<0 && i2>=0) {
nums1[i] = nums2[i2];
i2--;
continue;
}
if (nums1[i1] >= nums2[i2]) {
nums1[i] = nums1[i1];
i1--;
} else {
nums1[i] = nums2[i2];
i2--;
}
}
}
215 Kth Largest Element in an Array
https://leetcode.com/problems/kth-largest-element-in-an-array/
Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.
public int findKthLargest(int[] nums, int k) {
int r = nums.length-1;
k = nums.length - k + 1;
return findKthLargestRange(nums, k, 0 ,r);
}
private int findKthLargestRange(int[] nums, int k, int l , int r) {
int mid = l + (r-l)/2;
swap(l, mid, nums);
int i=l+1;
int j=r;
int val = nums[l];
while (i<=j) {
if (nums[i]>val) {
swap(i, j, nums);
j--;
} else {
i++;
}
}
swap(j, l ,nums);
if (j==k-1) {
return nums[j];
} else if (j<k-1) {
return findKthLargestRange(nums, k, j+1 ,r);
} else {
return findKthLargestRange(nums, k, l ,j-1);
}
}
private void swap(int i, int j, int[] arr) {
int c = arr[i];
arr[i] = arr[j];
arr[j] = c;
}
對撞指針
167 Two Sum II - Input array is sorted
https://leetcode.com/problems/two-sum-ii-input-array-is-sorted
Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.
public int[] twoSum(int[] numbers, int target) {
int lft = 0;
int rgt = numbers.length-1;
while(lft < rgt){
if(numbers[lft] + numbers[rgt] > target){
rgt--;
} else if(numbers[lft] + numbers[rgt] < target){
lft++;
} else {
int[] ans = {lft+1, rgt+1};
return ans;
}
}
throw new RuntimeException("There is no answer such that they add up to a specific target number.");
}
125 Valid Palindrome
https://leetcode.com/problems/valid-palindrome/
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
public boolean isPalindrome(String s) {
// 0-9 48-57
// A-Z 65-90
// a-z 97-122
int i = 0;
int j = s.length()-1;
String sb = s.toLowerCase();
while(i<j){
int charAtI = sb.charAt(i);
int charAtJ = sb.charAt(j);
if(charAtI<48 || (charAtI>57 && charAtI<97) || charAtI>122){
i++;
continue;
}
if(charAtJ<48 || (charAtJ>57 && charAtJ<97) || charAtJ>122){
j--;
continue;
}
if(charAtI == charAtJ){
i++;
j--;
}else{
return false;
}
}
return true;
}
345 Reverse Vowels of a String
https://leetcode.com/problems/reverse-vowels-of-a-string/
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
public String reverseVowels(String s) {
StringBuffer sb = new StringBuffer(s);
int i = 0;
int j = sb.length()-1;
Set<Character> vowels = new HashSet<>();
vowels.add('A');
vowels.add('E');
vowels.add('I');
vowels.add('O');
vowels.add('U');
vowels.add('a');
vowels.add('e');
vowels.add('i');
vowels.add('o');
vowels.add('u');
while(i<j){
char charAtI = sb.charAt(i);
char charAtJ = sb.charAt(j);
if(vowels.contains(charAtI) && vowels.contains(charAtJ)){
sb.setCharAt(i, charAtJ);
sb.setCharAt(j, charAtI);
i++;
j--;
} else if(vowels.contains(charAtI)) {
j--;
} else if(vowels.contains(charAtJ)) {
i++;
} else {
i++;
j--;
}
}
return sb.toString();
}
11 Container With Most Water
https://leetcode.com/problems/container-with-most-water/
Given n non-negative integers a1, a2, …, an , where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of line i is at (i, ai) and (i, 0). Find two lines, which together with x-axis forms a container, such that the container contains the most water.
public int maxArea(int[] height) {
int area = 0;
int lft = 0;
int rgt = height.length-1;
while(lft<rgt){
int h = height[lft]<height[rgt]?height[lft]:height[rgt];
int w = rgt-lft;
area = h*w>area?h*w:area;
if(height[lft]<height[rgt]){
lft++;
} else {
rgt--;
}
}
return area;
}
滑動窗口
209 Minimum Size Subarray Sum
https://leetcode.com/problems/minimum-size-subarray-sum/
Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. If there isn’t one, return 0 instead.
public int minSubArrayLen(int s, int[] nums) {
// 區間[lft,rgt]
int lft = 0;
int rgt = -1;
int minSize = nums.length + 1;
int sum = 0;
while(lft<nums.length-1){
if(rgt<nums.length-1 && sum<s){
rgt++;
sum += nums[rgt];
}else{
if(sum>=s){
minSize = (rgt-lft+1)<minSize?(rgt-lft+1):minSize;
}
sum -= nums[lft];
lft++;
}
}
if(minSize == nums.length + 1){
return 0;
}
return minSize;
}
3 Longest Substring Without Repeating Characters
https://leetcode.com/problems/longest-substring-without-repeating-characters/
Given a string, find the length of the longest substring without repeating characters.
public int lengthOfLongestSubstring(String s) {
Set<Character> set = new HashSet<>();
int i=0;
int j=0;
int l=0;
while (i<s.length() && j<s.length()) {
if (!set.contains(s.charAt(i))) {
set.add(s.charAt(i++));
l=Math.max(l,i-j);
} else {
set.remove(s.charAt(j++));
}
}
return l;
}
438 Find All Anagrams in a String
https://leetcode.com/problems/find-all-anagrams-in-a-string
Given a string s and a non-empty string p, find all the start indices of p’s anagrams in s.
public List<Integer> findAnagrams(String s, String p) {
List<Integer> list = new ArrayList<>();
if (s==null || s.length()==0 || p==null || p.length()==0) {
return list;
}
char[] hash = new char[256];
for (char c : p.toCharArray()) {
hash[c]++;
}
int right=0, left=0, count=0;
while(right<s.length()) {
if (hash[s.charAt(right)]>0) {
hash[s.charAt(right)]--;
right++;
count++;
} else {
hash[s.charAt(left)]++;
left++;
count--;
}
if (count == p.length()) {
list.add(left);
}
}
return list;
}
76 Minimum Window Substring
https://leetcode.com/problems/minimum-window-substring/
Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).
Hard模式，也可以用滑動窗口解，但因爲我有Hard恐懼症，暫時沒做。