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Update 0454.四数相加II.md

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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Original file line number	Diff line number	Diff line change
`@@ -97,26 +97,25 @@ Java:`
`97`	`97`	```Java
`98`	`98`	`class Solution {`
`99`	`99`	`public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {`
`100`		`- Map<Integer, Integer> map = new HashMap<>();`
`101`		`- int temp;`
`102`	`100`	`int res = 0;`
	`101`	`+ Map<Integer, Integer> map = new HashMap<Integer, Integer>();`
`103`	`102`	`//统计两个数组中的元素之和,同时统计出现的次数,放入map`
`104`	`103`	`for (int i : nums1) {`
`105`	`104`	`for (int j : nums2) {`
`106`		`- temp = i + j;`
`107`		`- if (map.containsKey(temp)) {`
`108`		`- map.put(temp, map.get(temp) + 1);`
	`105`	`+ int tmp = map.getOrDefault(i + j, 0);`
	`106`	`+ if (tmp == 0) {`
	`107`	`+ map.put(i + j, 1);`
`109`	`108`	`} else {`
`110`		`- map.put(temp, 1);`
	`109`	`+ map.replace(i + j, tmp + 1);`
`111`	`110`	`}`
`112`	`111`	`}`
`113`	`112`	`}`
`114`	`113`	`//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`
`115`	`114`	`for (int i : nums3) {`
`116`	`115`	`for (int j : nums4) {`
`117`		`- temp = i + j;`
`118`		`- if (map.containsKey(0 - temp)) {`
`119`		`- res += map.get(0 - temp);`
	`116`	`+ int tmp = map.getOrDefault(0 - i - j, 0);`
	`117`	`+ if (tmp != 0) {`
	`118`	`+ res += tmp;`
`120`	`119`	`}`
`121`	`120`	`}`
`122`	`121`	`}`

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