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leetcode 150. Evaluate Reverse Polish Notation ------ java
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
就是求逆波兰表达式（后续遍历）的结果。
1、直接求解，很慢
public class Solution { public int evalRPN(String[] tokens) { int len = tokens.length; if( len == 0) return 0; for( int i = 0 ; i < len ; i ++ ){ if( tokens[i].equals("+") ) helper(tokens,i,1); else if( tokens[i].equals("-") ) helper(tokens,i,2); else if( tokens[i].equals("*") ) helper(tokens,i,3); else if( tokens[i].equals("/") ) helper(tokens,i,4); } return Integer.valueOf(tokens[0]); } public void helper(String[] tokens,int pos,int type){ int pos1 = -1,pos2 = 0 ; tokens[pos] = null; while( pos >= 0 ){ if( tokens[pos] != null){ if( pos1 == -1) pos1 = pos; else{ pos2 = pos; break; } } pos--; } int num1 = Integer.valueOf(tokens[pos1]); int num2 = Integer.valueOf(tokens[pos2]); if( type == 1){ tokens[pos2] = String.valueOf(num1+num2); }else if( type == 2){ tokens[pos2] = String.valueOf(num2-num1); }else if( type == 3){ tokens[pos2] = String.valueOf(num2*num1); }else if( type == 4){ tokens[pos2] = String.valueOf(num2/num1); } tokens[pos1] = null; } }
2、使用栈，很简单。
public class Solution { public int evalRPN(String[] tokens) { int len = tokens.length; if( len == 0) return 0; Stack<Integer> stack = new Stack<Integer>(); for( int i = 0 ; i < len ; i ++ ){ if( tokens[i].equals("+") ){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num1+num2); } else if( tokens[i].equals("-") ){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num2-num1); } else if( tokens[i].equals("*") ){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num1*num2); } else if( tokens[i].equals("/") ){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num2/+num1); }else{ stack.push(Integer.valueOf(tokens[i])); } } return stack.pop(); } }
leetcode 150. Evaluate Reverse Polish Notation ------ java
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