LeetCode 20：有效的括號

給定一個只包含'(',')','[','],'{','}'的字符串，判斷字符串是否有效。
示例：
輸入:"()", true
輸入:"()[]{}", true
輸入:"(]", false
輸入:"([)]", false
輸入:"([])", true

 

時間複雜度：O（n），對字符串執行了一個for循環遍歷。

NSString *string = @"{()}";
NSMutableArray *array = [NSMutableArray array];
BOOL isVaild = YES;
for (int i = 0; i < string.length ; i++) {
    NSString *subString = [string substringWithRange:NSMakeRange(i, 1)];
    if ([subString isEqualToString:@"("] || [subString isEqualToString:@"["] || [subString isEqualToString:@"{"]) {
        [array addObject:subString];
    } else {
        if (array.count) {
            NSString *leftValue = [array objectAtIndex:array.count - 1];
            if (
                ([leftValue isEqualToString:@"("] && ![subString isEqualToString:@")"]) ||
                ([leftValue isEqualToString:@"{"]  && ![subString isEqualToString:@"}"]) ||
                ([leftValue isEqualToString:@"["]  && ![subString isEqualToString:@"]"])
                ) {
                isVaild = NO;
                break;
            }
            [array removeLastObject];
        } else {
            isVaild = NO;
            break;
        }
    }
}
if (array.count) {
    isVaild = NO;
}