8. String to Integer (atoi)

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LeetCode

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class Solution {
    private boolean is_num(char ch){		
		if((ch-'0'<=9)&&(ch-'0'>=0)) return true;
		return false;
	}
	public int myAtoi(String str) {
        // str is empty 
		if(str==null||str.length()==0) return 0;	
        
		// str contains only whitespace characters	
		String newStr=str.trim();
        if(newStr.length()==0) return 0;
        
        // not a plus or minus sign or numerical digit at the head
		char ch=newStr.charAt(0);
	    if(ch!='-'&&ch!='+'&&!is_num(ch)) return 0;
        // only plus or minus sign
        if(ch=='-'&&newStr.length()==1||ch=='+'&&newStr.length()==1) return 0;
		
        // multiple non-numberical digits except the plus and minus sign before integer numbers
		int count=0;
		while (count<newStr.length()&&!is_num(newStr.charAt(count))) {
			count++;
		}
		if(count>1) return 0;
			
		boolean sign=true;
		StringBuilder sb=new StringBuilder();
		
		if(newStr.charAt(0)=='-'){
			sign=false;
			for(int i=1;i<newStr.length();i++){
					if(is_num(newStr.charAt(i)))
						sb.append(newStr.charAt(i));
					else break;
				}
		} else if(newStr.charAt(0)=='+'){
			for(int i=1;i<newStr.length();i++){
					if(is_num(newStr.charAt(i)))
						sb.append(newStr.charAt(i));
					else break;
				}
		} else {
			for(int i=0;i<newStr.length();i++){
					if(is_num(newStr.charAt(i)))
						sb.append(newStr.charAt(i));
					else break;
			}
		}
		
        // e.g. "+0000" or "-0000" or "0000"
		while (sb.charAt(0)=='0') {
			sb.deleteCharAt(0);
			if(sb.length()==0) return 0;
		}
        
        // if the number is too big, like bigger than maximum long integer.
		if(sign&&sb.length()>10) return Integer.MAX_VALUE;
		if(!sign&&sb.length()>10) return Integer.MIN_VALUE;
		
        // to check whether integer is within the 32-bit signed integer range: [−2^31,  2^31 − 1]
		if(!sign) return (0-Long.valueOf(sb.toString()))<=Integer.MIN_VALUE ?Integer.MIN_VALUE:0-Integer.valueOf(sb.toString());
		return Long.valueOf(sb.toString())>=Integer.MAX_VALUE?Integer.MAX_VALUE:Integer.valueOf(sb.toString());
	}
}
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