一、两数之和

1、题目链接

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2、代码

class Solution {public int[] twoSum(int[] nums, int target) {for(int i = 0; i < nums.length; ++i){for(int j = i+1; j < nums.length; ++j){if(nums[i] + nums[j] == target){return new int[]{i, j};}}}return new int[]{0, 0};}
}

二、回文数

1、题目链接

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2、代码

class Solution {public boolean isPalindrome(int x) {if(x < 0 || (x % 10 == 0 && x != 0)){return false;}int reverseNumber = 0;while(x > reverseNumber){reverseNumber = reverseNumber * 10 + x % 10;x /= 10;}return reverseNumber / 10 == x || reverseNumber == x;}
}

三、罗马数字转整数

1、题目链接

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2、代码

class Solution {Map<Character, Integer> symbplValues = new HashMap<Character, Integer>(){{put('I', 1);put('V', 5);put('X', 10);put('L', 50);put('C', 100);put('D', 500);put('M', 1000);}};public int romanToInt(String s) {int res = 0;int n = s.length();for(int i = 0; i < n; ++i){int num = symbplValues.get(s.charAt(i));if(i == n - 1){res += num;continue;}int num1 = symbplValues.get(s.charAt(i+1));if(num1 > num){res += (num1 - num);++i; } else{res += num;}}return res;}
}

四、最长公共前缀

1、题目链接

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2、代码

class Solution {public String longestCommonPrefix(String[] strs) {if(strs == null || strs.length == 0){return "";}String res = new String();int max_length = 1000000;for(int i = 0; i < strs.length; ++i){max_length = Math.min(max_length, strs[i].length());}for(int i = 0; i < max_length; ++i){char ch = strs[0].charAt(i);for(int j = 1; j < strs.length; ++j){if(strs[j].charAt(i) != ch){return res;}}res += ch;}return res;}
}

五、有效的括号

1、题目链接

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2、代码

class Solution {public boolean isValid(String s) {int n = s.length();if((n & 1) == 1){return false;}Deque<Character> stack = new LinkedList<Character>();for(int i = 0; i < n; ++i){if(stack.isEmpty()){stack.push(s.charAt(i));} else{if((stack.peek() == '(' && s.charAt(i) == ')')|| (stack.peek() == '[' && s.charAt(i) == ']')|| (stack.peek() == '{' && s.charAt(i) == '}')){stack.pop(); } else{stack.push(s.charAt(i));}}}return stack.isEmpty();}
}

六、合并两个有序链表

1、题目链接

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2、代码

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode mergeTwoLists(ListNode list1, ListNode list2) {if(list1 == null){return list2;} else if(list2 == null){return list1;} else if(list1.val < list2.val){list1.next = mergeTwoLists(list1.next, list2);return list1;} else{list2.next = mergeTwoLists(list2.next, list1);return list2;}}
}

七、删除有序数组中的重复项

1、题目链接

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2、代码

class Solution {public int removeDuplicates(int[] nums) {int left = 0;int right = 0;int n = nums.length;while(right < n){while(right < n - 1 && nums[right] == nums[left]){++right;}if(right == n - 1 && nums[left] == nums[right]){break;}++left;nums[left] = nums[right];      }return left + 1;}
}

八、移除元素

1、题目链接

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2、代码

class Solution {public int removeElement(int[] nums, int val) {int left = 0;int right = 0;int n = nums.length;while(right < n){if(nums[right] == val){++right;continue;} else{nums[left] = nums[right];++left;++right;}}return left;}
}

九、找出字符串中第一个匹配项的下标

1、题目链接

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2、代码

class Solution {public int strStr(String haystack, String needle) {for(int i = 0; i < haystack.length(); ++i){int flag = 0;if(i + needle.length() > haystack.length()){break;}for(int j = 0; j < needle.length(); ++j){if(haystack.charAt(i + j) != needle.charAt(j)){flag = -1;break;}}if(flag == 0){return i;}}return -1;}
}

十、搜索插入位置

1、题目链接

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2、代码

class Solution {public int searchInsert(int[] nums, int target) {int left = 0;int right = nums.length - 1;int ans = -1;while(left <= right){int mid = ((right - left) >> 1) + left;if(nums[mid] == target){return mid;} else if(nums[mid] < target){left = mid + 1; } else{ans = mid;right = mid - 1;}}if(ans == -1){return nums.length;}return ans;}
}

十一、最后一个单词的长度

1、题目链接

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2、代码

class Solution {public int lengthOfLastWord(String s) {int len = 0;int flag = 0;for(int i = 0; i < s.length(); ++i){if(s.charAt(i) == ' '){flag = 1;} else{if(flag == 1){len = 1;flag = 0;} else{++len;}}}return len;}
}

十二、加一

1、题目链接

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2、代码

class Solution {public int[] plusOne(int[] digits) {int n = digits.length;for(int i = n - 1; i >= 0; --i){digits[i] = (digits[i] + 1) % 10;if(digits[i] != 0){return digits;}}digits = new int[digits.length + 1];digits[0] = 1;return digits;}
}

十三、二进制求和

1、题目链接

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2、代码

class Solution {public String addBinary(String a, String b) {StringBuffer ans = new StringBuffer();int i = a.length() - 1;int j = b.length() - 1;int carry = 0;while(i >= 0 && j >= 0){int num1 = a.charAt(i) - '0';int num2 = b.charAt(j) - '0';int num = (num1 + num2 + carry) % 2;carry = (num1 + num2 + carry) / 2;ans.append(num);--i;--j;}while(i >= 0){int num1 = a.charAt(i) - '0';int num = (num1 + carry) % 2;carry = (num1 + carry) / 2;ans.append(num);--i;}while(j >= 0){int num2 = b.charAt(j) - '0';int num = (num2 + carry) % 2;carry = (num2 + carry) / 2;ans.append(num);--j;}if(carry == 1){ans.append(1);}ans.reverse();return ans.toString();}
}

十四、x 的平方根

1、题目链接

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2、代码

class Solution {public int mySqrt(int x) {int left = 0;int right = x;int ans = -1;while(left <= right){int mid = ((right - left) >> 1) + left;long num = (long)mid * mid;if(num == x){return mid;} else if(num < x){ans = mid;left = mid + 1;} else{right = mid - 1;}}return ans;}
}

十五 爬楼梯

1、题目链接

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2、代码

class Solution {public int climbStairs(int n) {int []dp = new int[n+1];dp[0] = 1;dp[1] = 1;for(int i = 2; i <= n; ++i){dp[i] = dp[i-1] + dp[i-2];}return dp[n];}
}