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Evaluate Reverse Polish Notation
Evaluate Reverse Polish Notation
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9 ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
这道题还是比较Easy，直接用堆栈，遇到数字就进栈，遇到运算符弹出两个数，将运算结果入栈。最后返回栈底元素就okay了
public class Solution { public int evalRPN(String[] tokens) { int top = -1;//栈指针 int stack[] = new int[tokens.length / 2 + 1]; for(int i = 0; i < tokens.length; i++){ char temp = tokens[i].charAt(0); if(temp != ‘+‘ && !(tokens[i].length() == 1 && temp == ‘-‘) && temp != ‘*‘ && temp != ‘/‘){//数字直接入栈 stack[++top] = Integer.valueOf(tokens[i]); } else{//元素符，弹两个数字出来计算 int num1; int num2; int result = 0; num2 = stack[top--]; num1 = stack[top--]; switch(temp){ case ‘+‘: result = num1 + num2; break;//入栈 case ‘-‘: result = num1 - num2; break; case ‘*‘: result = num1 * num2; break; case ‘/‘: result = num1 / num2; break; } stack[++top] = result;//结果入栈 } } return stack[top]; }}
Evaluate Reverse Polish Notation
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