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Update 0015.三数之和.md

增加了java语言版本的哈希解法

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`‎problems/0015.三数之和.md‎`

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`@@ -256,7 +256,7 @@ while (right > left) {`
`256`	`256`	`## 其他语言版本`
`257`	`257`
`258`	`258`	`### Java:`
`259`		`-`
	`259`	`+(版本一) 双指针`
`260`	`260`	```Java
`261`	`261`	`class Solution {`
`262`	`262`	`public List<List<Integer>> threeSum(int[] nums) {`
`@@ -297,7 +297,43 @@ class Solution {`
`297`	`297`	`}`
`298`	`298`	`}`
`299`	`299`	```
	`300`	`+(版本二) 使用哈希集合`
	`301`	+```Java
	`302`	`+class Solution {`
	`303`	`+ public List<List<Integer>> threeSum(int[] nums) {`
	`304`	`+ List<List<Integer>> result = new ArrayList<>();`
	`305`	`+ Arrays.sort(nums);`
`300`	`306`
	`307`	`+ for (int i = 0; i < nums.length; i++) {`
	`308`	`+ // 如果第一个元素大于零,不可能凑成三元组`
	`309`	`+ if (nums[i] > 0) {`
	`310`	`+ return result;`
	`311`	`+ }`
	`312`	`+ // 三元组元素a去重`
	`313`	`+ if (i > 0 && nums[i] == nums[i - 1]) {`
	`314`	`+ continue;`
	`315`	`+ }`
	`316`	`+`
	`317`	`+ HashSet<Integer> set = new HashSet<>();`
	`318`	`+ for (int j = i + 1; j < nums.length; j++) {`
	`319`	`+ // 三元组元素b去重`
	`320`	`+ if (j > i + 2 && nums[j] == nums[j - 1] && nums[j - 1] == nums[j - 2]) {`
	`321`	`+ continue;`
	`322`	`+ }`
	`323`	`+`
	`324`	`+ int c = -nums[i] - nums[j];`
	`325`	`+ if (set.contains(c)) {`
	`326`	`+ result.add(Arrays.asList(nums[i], nums[j], c));`
	`327`	`+ set.remove(c); // 三元组元素c去重`
	`328`	`+ } else {`
	`329`	`+ set.add(nums[j]);`
	`330`	`+ }`
	`331`	`+ }`
	`332`	`+ }`
	`333`	`+ return result;`
	`334`	`+ }`
	`335`	`+}`
	`336`	+```
`301`	`337`	`### Python:`
`302`	`338`	`(版本一) 双指针`
`303`	`339`

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