Algorithms—18.4Sum

思路：跟三和问题本质一样，加上一层循环，两边设置端点，然后内部双端扫描

public class Solution {
List<List<Integer>> ans=new ArrayList<List<Integer>>();
public List<List<Integer>> fourSum(int[] nums, int target) {
if (nums.length<4) {
return ans;
}
Arrays.sort(nums);
int length=nums.length;
for (int i = 0; i <length-3; i++) {
for (int j = length-1; j >=i+3; j--) {
if ((i==0||nums[i]!=nums[i-1])&&(j==length-1||nums[j]!=nums[j+1])) {
f(nums,target,i,i+1,j-1,j);
}
}
}
return ans;
}
public void f(int[] nums, int target,int s,int m,int n,int e){
int is=nums[s];
int ie=nums[e];
while (m<n) {
int res=is+nums[m]+nums
+ie-target;
if (res==0) {
List<Integer> list=new ArrayList<Integer>();
list.add(is);
list.add(nums[m]);
list.add(nums
);
list.add(ie);
ans.add(list);
while (m<n&&nums[m]==nums[m+1]) {
m++;
}
while (m<n&&nums
==nums[n-1]) {
n--;
}
m++;
n--;
}else if(res<0){
m++;
}else{
n--;
}
}
}
}