遍历技巧:2,3,4sum&3sum closest&set matrix zeroes&Container With Most Water

2 sum

public class Solution {    public int[] twoSum(int[] nums, int target) {        int[] res = new int[2];        HashMap<Integer,Integer> map = new HashMap();        for(int i=0;i<nums.length;i++){            if(map.containsKey(target-nums[i])){                res[1] = i+1;                res[0] = map.get(target-nums[i]);                return res;            }            map.put(nums[i],i+1);        }        return res;    }}

3 sum

public List<List<Integer>> threeSum(int[] nums) {        Arrays.sort(nums);        List<List<Integer>> res = new LinkedList();        for(int i=0;i<nums.length-2;i++){            if(i>0&&nums[i]==nums[i-1]) continue;            int lo=i+1,hi=nums.length-1;            while(lo<hi){                int sum=nums[i]+nums[lo]+nums[hi];                if(sum==0){                    res.add(Arrays.asList(nums[i],nums[lo],nums[hi]));                    while(lo<hi&&nums[lo]==nums[lo+1])lo++;                    while(lo<hi&&nums[hi]==nums[hi-1])hi--;                    lo++;                    hi--;                }else if(sum<0) lo++;                else hi--;            }        }        return res;    }

4 sum

public List<List<Integer>> fourSum(int[] nums, int target) {        List<List<Integer>> res = new ArrayList();        if(nums.length<4) return res;                Arrays.sort(nums);        for(int i=0;i<nums.length-3;i++){            if(i>0&&nums[i]==nums[i-1]) continue;            for(int j=i+1;j<nums.length-2;j++){                if(j>i+1&&nums[j]==nums[j-1]) continue;                int lo=j+1,hi=nums.length-1;                while(lo<hi){                    int sum = nums[i]+nums[j]+nums[lo]+nums[hi];                    if(sum==target){                        res.add(Arrays.asList(nums[i],nums[j],nums[lo],nums[hi]));                        while(lo<hi&&nums[lo]==nums[lo+1]) lo++;                        while(lo<hi&&nums[hi]==nums[hi-1]) hi--;                        lo++;                        hi--;                    }else if(sum<target) lo++;                    else hi--;                }            }        }        return res;    }

3 sum closest

public int threeSumClosest(int[] nums, int target) {        int res = nums[0] + nums[1] + nums[2];        Arrays.sort(nums);        for(int i=0;i<nums.length-2;i++){            int lo=i+1,hi=nums.length-1;            while(lo<hi){                int sum = nums[i]+nums[lo]+nums[hi];                if(sum<target) lo++;                else hi--;                if(Math.abs(sum-target)<Math.abs(res-target)) res = sum;            }        }        return res;    }

set matrix zeroes

public void setZeroes(int[][] matrix) {        boolean fr = false;        boolean fc = false;        for(int i=0;i<matrix.length;i++){            for(int j=0;j<matrix[0].length;j++){                if(matrix[i][j]==0){                    if(i==0) fr = true;                    if(j==0) fc = true;                    matrix[0][j]=0;                    matrix[i][0]=0;                }            }        }        for(int i=1;i<matrix.length;i++){            for(int j=1;j<matrix[0].length;j++){                if(matrix[0][j]==0||matrix[i][0]==0){                    matrix[i][j]=0;                }            }        }        if(fr){            for(int i=0;i<matrix[0].length;i++){                matrix[0][i] =0;            }        }        if(fc){            for(int j=0;j<matrix.length;j++){                matrix[j][0] =0;            }        }    }

Container With Most Water

public int maxArea(int[] height) {        int left = 0;        int right = height.length - 1;        int max = 0;        while(left < right)        {            max = Math.max(max,area(right,left,height));            if(height[left]< height[right])            {                left++;            }else            {                right--;            }        }        return max;    }    private int area(int right,int left,int[] height)    {        return (right - left) * Math.min(height[right],height[left]);    }