B站的笔试题，题目我就不写了，直接贴代码
第一题：递归
boolean result = false;
public boolean Game24Points (int[] arr) {
if(arr == null || arr.length == 0) {
return false;
}
f(arr, 1, arr[0]);
return result;
}
public void f(int[] arr, int index,int sum) {
if(index >= arr.length || result == true) {
if(sum == 24) {
result = true;
}
return;
}
f(arr, index+1, sum + arr[index]);
f(arr, index+1, sum - arr[index]);
f(arr, index+1, sum * arr[index]);
if(arr[index]!= 0) {
f(arr, index+1, sum / arr[index]);
}
}
第二题：利用栈
public boolean IsValidExp (String s) {
Stack<Character> stack = new Stack<>();
for(int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
switch (ch) {
case '{':
stack.push('{');
break;
case '[':
stack.push('[');
break;
case '(':
stack.push('(');
break;
case '}':
{
if(stack.isEmpty()) {
return false;
}
if(stack.peek() == '{') {
stack.pop();
} else {
return false;
}
}
break;
case ']':
{
if(stack.isEmpty()) {
return false;
}
if(stack.peek() == '[') {
stack.pop();
} else {
return false;
}
}
break;
case ')':
{
if(stack.isEmpty()) {
return false;
}
if(stack.peek() == '(') {
stack.pop();
} else {
return false;
}
}
break;
default:
break;
}
}
if(stack.isEmpty()) {
return true;
} else {
return false;
}
}
第三题：贪心
public int GetCoinCount (int N) {
int money = 1024 - N;
int count = 0;
count += money/64;
money -= (64 * (money/64));
count += money / 16;
money -= (16 * (money/16));
count += money / 4;
money -= (4 * (money/4));
count += money / 1;
return count;
}
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