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Update 0454.四数相加II.md

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原先是先判断map中是否有这个key，后面更新时再get(key)，经过测试，直接使用getOrDefault()方法效率会更高一些，没有当前key时返回0，值为0就是没有当前key
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problems/0454.四数相加II.md
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@ -97,26 +97,25 @@ Java：
```Java
class Solution {
public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
Map<Integer, Integer> map = new HashMap<>();
int temp;
int res = 0;
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
//统计两个数组中的元素之和，同时统计出现的次数，放入map
for (int i : nums1) {
for (int j : nums2) {
temp = i + j;
if (map.containsKey(temp)) {
map.put(temp, map.get(temp) + 1);
int tmp = map.getOrDefault(i + j, 0);
if (tmp == 0) {
map.put(i + j, 1);
} else {
map.put(temp, 1);
map.replace(i + j, tmp + 1);
}
}
}
//统计剩余的两个元素的和，在map中找是否存在相加为0的情况，同时记录次数
for (int i : nums3) {
for (int j : nums4) {
temp = i + j;
if (map.containsKey(0 - temp)) {
res += map.get(0 - temp);
int tmp = map.getOrDefault(0 - i - j, 0);
if (tmp != 0) {
res += tmp;
}
}
}