20. 有效的括号

参考代码

#include <stack>class Solution {
public:bool isValid(string s) {if(s.size() < 2){ //特判：空字符串和一个字符的情况return false;}bool flag = true;stack<char> st; //栈for(int i=0; i<s.size(); i++){if(s[i] == '(' || s[i]=='{' || s[i]=='['){st.push(s[i]);}if(s[i] == ')' || s[i]=='}' || s[i]==']'){if(st.empty()){flag = false;break;}char c = st.top();//括号匹配if((c == '(' && s[i] == ')') || (c == '{' && s[i]=='}') || (c =='[' && s[i]==']')){st.pop();}else{flag = false;break;}}}if(!st.empty()) //栈里面还有元素{flag = false;}return flag;}
};

2. 两数相加

参考代码

/*** Definition for singly-linked list.* struct ListNode {*     int val;*     ListNode *next;*     ListNode() : val(0), next(nullptr) {} 初始化*     ListNode(int x) : val(x), next(nullptr) {} 赋值*     ListNode(int x, ListNode *next) : val(x), next(next) {}* };*/
class Solution {
public:ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {ListNode *previous, *current;previous = new ListNode(-1); //虚拟结点current = previous;int t = 0; //注意进位while(l1 || l2 || t){if(l1){t += l1->val;l1 = l1->next;}if(l2){t += l2->val;l2 = l2->next;}current = current->next = new ListNode(t % 10);t /= 10;}//返回头结点的下一个结点，即第一个数字结点return previous->next;}
};

228. 汇总区间

参考代码

class Solution {
public:vector<string> summaryRanges(vector<int>& nums) {vector<string> result;int i=0, size=nums.size();while(i < size){int low = i;i++;while(i < size && nums[i] == nums[i-1] + 1){i++;}int high = i-1;//找不到连续数字：string s = to_string(nums[low]);if(low < high){ //形成区间s += "->" + to_string(nums[high]);}//否则，区间只有一个数result.push_back(s);}return result;}
};