Problem:
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.
Analysis:
Use a stack to help. If the element is '(', '[', '{', push it onto stack. If the element is ')', ']', '}', pop one element from stack and see if they match with each other. If not match, then return false;
Other cases may cause false result are: 1. Meet with ')', ']', '}', but the stack is empty; 2. The string has been processed, but the stack is not empty.
The time complexity is O(n), the space complexity is O(n) in worst case, O(1) in best case;
Code:
1 public class Solution { 2 public boolean isValid(String s) { 3 // Start typing your Java solution below 4 // DO NOT write main() function 5 char[] stack = new char[100]; 6 int tos = 0; 7 char tmp; 8 9 for (int i=0; i<s.length(); ++i) { 10 tmp = s.charAt(i); 11 switch(tmp) { 12 case '(': case '[': case '{': stack[tos++] = tmp; break; 13 case ')': 14 if (tos==0 || stack[--tos]!='(') 15 return false; 16 break; 17 case '}': 18 if (tos==0 || stack[--tos]!='{') 19 return false; 20 break; 21 case ']': 22 if (tos==0 || stack[--tos]!='[') 23 return false; 24 break; 25 } 26 } 27 28 if (tos != 0) 29 return false; 30 else 31 return true; 32 } 33 }
Attention: