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  • 3sum-closest

    https://leetcode.com/problems/3sum-closest/

    // At first, my DP solution exceeds time limitation
    // Then with the hint fo the discussion board,
    // I have the final solution of impl2,
    // which is very smart!

    class Solution {
        map<pair<int, pair<int, int>>, int> mp;
        int diff(int a, int b) {
            return a>=b ? a-b : b-a;
        }
        
        bool close(int a, int b) {
            if (b == 0) {
                // for init condition
                return true;
            }
            
            if (a < 0) {
                a *= -1;
            }
            if (b < 0) {
                b *= -1;
            }
            return a < b;
        }
        
        vector<int> nums;
        
    public:
        int threeSumClosest(vector<int>& n, int target) {
            nums = n;
            
            int vlen = nums.size();
            if (vlen < 3) {
                return 0;
            }
            
            //return impl(target, vlen, 3);
            return impl2(target, vlen);
        }
        
        int impl2(const int &target, const int &end) {
            sort(nums.begin(), nums.end());
            // can be 0, as if 0 returns asap
            int df = 0;
    
            int n;        
            int p;
            int q;
            int ndf;
            for (int i = 0; i < end-2; i++) {
                n = nums[i];
                p = i + 1;
                q = end - 1;
                while (p < q) {
                    ndf = n + nums[p] + nums[q];
                    if (ndf == target) {
                        return target;
                    }
                    else if (ndf > target) {
                        q--;
                    }
                    else {
                        p++;
                    }
                    if (close(ndf - target, df)) {
                        df = ndf - target;
                    }
                }
            }
            return target + df;
        }
        
        // below solution exceed time limit
        int impl(const int &target, const int &end, const int &sz) {
            if (sz == 0 || end == 0) {
                return 0;
            }
            
            pair<int, pair<int, int>> pr = make_pair(target, make_pair(end, sz));
            if (mp.find(pr) != mp.end()) {
                return mp[pr];
            }
            
            int ret;
            int df;
            
            if (sz == 1) {
                
                ret = nums[0];
                df = diff(ret, target);
                
                for (int i=1; i<end; i++) {
                    if (diff(nums[i], target) < df) {
                        ret = nums[i];
                        df = diff(ret, target);
                    }
                }
                // return
            }
            else {
                if (end < sz) {
                    return 0;
                }
                else if (end == sz) {
                    ret = 0;
                    for (int i=0; i<sz; i++) {
                        ret += nums[i];
                    }
                    // return
                }
                else {
                    
                    ret = impl(target-nums[end-1], end-1, sz-1) + nums[end-1];
                    df = diff(ret, target);
                    
                    int a = impl(target, end-1, sz);
                    if (diff(a, target) < df) {
                        ret = a;
                    }
                    //return
                }
            }
            
            mp[pr] = ret;
            return ret;
            
        }
        
    };
    
    // Below may be duplicate
    
    class Solution {
        map<pair<int, pair<int, int>>, int> mp;
        int diff(int a, int b) {
            return a>=b ? a-b : b-a;
        }
        
        bool close(int a, int b) {
            if (b == 0) {
                // for init condition
                return true;
            }
            
            if (a < 0) {
                a *= -1;
            }
            if (b < 0) {
                b *= -1;
            }
            return a < b;
        }
        
        vector<int> nums;
        
    public:
        int threeSumClosest(vector<int>& n, int target) {
            nums = n;
            
            int vlen = nums.size();
            if (vlen < 3) {
                return 0;
            }
            
            //return impl(target, vlen, 3);
            return impl2(target, vlen);
        }
        
        int impl2(const int &target, const int &end) {
            sort(nums.begin(), nums.end());
            // can be 0, as if 0 returns asap
            int df = 0;
    
            int n;        
            int p;
            int q;
            int ndf;
            for (int i = 0; i < end-2; i++) {
                n = nums[i];
                p = i + 1;
                q = end - 1;
                while (p < q) {
                    ndf = n + nums[p] + nums[q];
                    if (ndf == target) {
                        return target;
                    }
                    else if (ndf > target) {
                        q--;
                    }
                    else {
                        p++;
                    }
                    if (close(ndf - target, df)) {
                        df = ndf - target;
                    }
                }
            }
            return target + df;
        }
        
        // below solution exceed time limit
        int impl(const int &target, const int &end, const int &sz) {
            if (sz == 0 || end == 0) {
                return 0;
            }
            
            pair<int, pair<int, int>> pr = make_pair(target, make_pair(end, sz));
            if (mp.find(pr) != mp.end()) {
                return mp[pr];
            }
            
            int ret;
            int df;
            
            if (sz == 1) {
                
                ret = nums[0];
                df = diff(ret, target);
                
                for (int i=1; i<end; i++) {
                    if (diff(nums[i], target) < df) {
                        ret = nums[i];
                        df = diff(ret, target);
                    }
                }
                // return
            }
            else {
                if (end < sz) {
                    return 0;
                }
                else if (end == sz) {
                    ret = 0;
                    for (int i=0; i<sz; i++) {
                        ret += nums[i];
                    }
                    // return
                }
                else {
                    
                    ret = impl(target-nums[end-1], end-1, sz-1) + nums[end-1];
                    df = diff(ret, target);
                    
                    int a = impl(target, end-1, sz);
                    if (diff(a, target) < df) {
                        ret = a;
                    }
                    //return
                }
            }
            
            mp[pr] = ret;
            return ret;
            
        }
        
    };
    
    
    // below is my original code some months ago, maybe wrong
    
        int sumClosest(vector<int>& nums, int target, int len) {
            if (len == 0) {
                return 0;
            }
            
            int vlen = nums.size();
            if (vlen < len) {
                return 0;
            }
            
            int i= 0;
            int result = 0;
            
            if (vlen == len) {
                for (i=0; i<vlen; i++) {
                    result += nums[i];
                }
                return result;
            }
            
            int tail = nums[vlen-1];
            
            if (vlen-1 == len) {
                for (i=0; i<vlen-1; i++) {
                    result += nums[i];
                }
            }
            else if (vlen-1 > len) {
                nums.pop_back();
                result = sumClosest(nums, target, len);
                nums.push_back(tail);
            }
            
            nums.pop_back();
            int ret = sumClosest(nums, target-tail, len-1) + tail;
            nums.push_back(tail);
            
            int diff1 = target - result;
            diff1 = (diff1>0)?diff1:diff1*-1;
            int diff2 = target - ret;
            diff2 = (diff2>0)?diff2:diff2*-1;
            if(diff1 < diff2) {
                return result;
            }
            return ret;
        }
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  • 原文地址:https://www.cnblogs.com/charlesblc/p/5408417.html
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