The question states:给出了指示（ U p， D 自己， L 向后， R 向右）在String moves中并从原点开始，最终结果将是原点吗？
一个明显的解决方案是简单地设置两个变量（用于跟踪垂直和水平运动），并查看所有方向操作后它们是否变为零：
public boolean judgeCircle(String moves) {
char[] chars = moves.toCharArray();
int vertical = 0;
int horizontal = 0;
for(int i = 0; i < chars.length; i++){
char c = chars[i];
switch(c){
case 'U':
vertical ++;
break;
case 'D':
vertical --;
break;
case 'L':
horizontal --;
break;
case 'R':
horizontal ++;
break;
}
}
return (vertical == 0) && (horizontal == 0);
}
对于Leetcode中的测试用例，该算法可在〜8ms 左右找到正确的解决方案。
但是，扫描所有操作4次的解决方案只能在〜1ms 中找到解决方案：
public boolean judgeCircle(String moves) {
int x = charCount(moves, 'R') - charCount(moves, 'L');
int y = charCount(moves, 'U') - charCount(moves, 'D');
return x == 0 && y == 0;
}
private int charCount(String moves, char c) {
int count = 0;
for(int i=0; i<moves.length(); i++) {
if(moves.charAt(i) == c) {
count++;
}
}
return count;
}
我将两种解决方案重复了大约10次，结果始终是一致的。但是，为什么扫描moves的算法运行速度比仅一次通过即可找到答案的算法快4倍（快4倍！）？
答案 0 :(得分：6)
我调整了您的示例，以便所有算法都在相同的数据上工作。我还在if实现中添加了另一个变体。
@State(Scope.Thread)
public class ForVsSwitch {
private static final int MOVES_LENGTH = 1024;
private static final char[] COMMANDS = { 'U', 'D', 'L', 'R'};
private char[] moves;
@Setup
public void prepare(){
Random random = new Random();
moves = new char[MOVES_LENGTH];
for(int i=0; i< MOVES_LENGTH; i++) {
moves[i] = COMMANDS[random.nextInt(4)];
}
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 3)
@Measurement(iterations = 5)
public void withSwitch() {
judgeCircleWithSwitch(moves);
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 3)
@Measurement(iterations = 5)
public void withFor() {
judgeCircleWithFor(moves);
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 3)
@Measurement(iterations = 5)
public void withIf() {
judgeCircleWithIf(moves);
}
private boolean judgeCircleWithSwitch(char[] moves) {
int vertical = 0;
int horizontal = 0;
for(int i = 0; i < moves.length; i++){
char c = moves[i];
switch(c){
case 'U':
vertical ++;
break;
case 'D':
vertical --;
break;
case 'L':
horizontal --;
break;
case 'R':
horizontal ++;
break;
}
}
return (vertical == 0) && (horizontal == 0);
}
private boolean judgeCircleWithIf(char[] moves) {
int vertical = 0;
int horizontal = 0;
for(int i = 0; i < moves.length; i++){
char c = moves[i];
if(c == 'U') {
vertical++;
} else if(c == 'D') {
vertical--;
} else if(c == 'L') {
horizontal--;
} else if(c == 'R') {
horizontal ++;
}
}
return (vertical == 0) && (horizontal == 0);
}
private boolean judgeCircleWithFor(char[] moves) {
int x = charCount(moves, 'R') - charCount(moves, 'L');
int y = charCount(moves, 'U') - charCount(moves, 'D');
return x == 0 && y == 0;
}
private int charCount(char[] moves, char c) {
int count = 0;
for(int i=0; i<moves.length; i++) {
if(moves[i] == c) {
count++;
}
}
return count;
}
}
如果我正确读取结果，则99.9％的执行速度要比27ms到29ms快，对吗？两种算法之间似乎没有什么区别。
Benchmark Mode Cnt Score Error Units
ForVsSwitch.withFor sample 5680658 0,003 ± 0,001 ms/op
ForVsSwitch.withFor:withFor·p0.00 sample 0,002 ms/op
ForVsSwitch.withFor:withFor·p0.50 sample 0,003 ms/op
ForVsSwitch.withFor:withFor·p0.90 sample 0,003 ms/op
ForVsSwitch.withFor:withFor·p0.95 sample 0,004 ms/op
ForVsSwitch.withFor:withFor·p0.99 sample 0,019 ms/op
ForVsSwitch.withFor:withFor·p0.999 sample 0,029 ms/op
ForVsSwitch.withFor:withFor·p0.9999 sample 0,075 ms/op
ForVsSwitch.withFor:withFor·p1.00 sample 2,912 ms/op
ForVsSwitch.withIf sample 8903669 0,002 ± 0,001 ms/op
ForVsSwitch.withIf:withIf·p0.00 sample 0,001 ms/op
ForVsSwitch.withIf:withIf·p0.50 sample 0,002 ms/op
ForVsSwitch.withIf:withIf·p0.90 sample 0,002 ms/op
ForVsSwitch.withIf:withIf·p0.95 sample 0,003 ms/op
ForVsSwitch.withIf:withIf·p0.99 sample 0,005 ms/op
ForVsSwitch.withIf:withIf·p0.999 sample 0,027 ms/op
ForVsSwitch.withIf:withIf·p0.9999 sample 0,051 ms/op
ForVsSwitch.withIf:withIf·p1.00 sample 5,202 ms/op
ForVsSwitch.withSwitch sample 8225249 0,002 ± 0,001 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.00 sample 0,001 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.50 sample 0,002 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.90 sample 0,002 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.95 sample 0,003 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.99 sample 0,018 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.999 sample 0,027 ms/op
ForVsSwitch.withSwitch:withSwitch·p0.9999 sample 0,071 ms/op
ForVsSwitch.withSwitch:withSwitch·p1.00 sample 22,610 ms/op
编辑：
我无法确认您的陈述是否成立。我简化了示例。我将静态列表用作两种算法的输入。我不进行热身，仅测量一次执行。不出所料，四遍比一遍昂贵。我真的不知道您的网站在衡量什么。
@State(Scope.Thread)
public class ForVsSwitch {
private char[] moves = {'U', 'D', 'L', ...};
@Benchmark
@BenchmarkMode(Mode.SingleShotTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 0)
@Measurement(iterations = 1, batchSize = 1)
@Fork(value = 1, warmups = 0)
public void withSwitch() {
judgeCircleWithSwitch();
}
@Benchmark
@BenchmarkMode(Mode.SingleShotTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 0)
@Measurement(iterations = 1, batchSize = 1)
@Fork(value = 1, warmups = 0)
public void withFor() {
judgeCircleWithFor();
}
private boolean judgeCircleWithSwitch() {
int vertical = 0;
int horizontal = 0;
for(int i = 0; i < moves.length; i++){
char c = moves[i];
switch(c){
case 'U':
vertical ++;
break;
case 'D':
vertical --;
break;
case 'L':
horizontal --;
break;
case 'R':
horizontal ++;
break;
}
}
return (vertical == 0) && (horizontal == 0);
}
private boolean judgeCircleWithFor() {
int x = charCount(moves, 'R') - charCount(moves, 'L');
int y = charCount(moves, 'U') - charCount(moves, 'D');
return x == 0 && y == 0;
}
private int charCount(char[] moves, char c) {
int count = 0;
for(int i=0; i<moves.length; i++) {
if(moves[i] == c) {
count++;
}
}
return count;
}
}
for循环比开关更昂贵。但是，正如其他评论所指出的那样，一次运行并不是可靠的性能分析。
Benchmark Mode Cnt Score Error Units
ForVsSwitch.withFor ss 0,577 ms/op
ForVsSwitch.withSwitch ss 0,241 ms/op