JS 二分法定位左右边界

递归查找

function erfen_digui(arr, val, left = 0, right = arr.length - 1) {
        if (left > right) {
          return -1;
        }
        let cent = Math.floor((right + left) / 2);
        if (arr[cent] === val) {
          return `最终查找结果下标为${cent}`;
        } else if (arr[cent] > val) {
          right = cent - 1;
        } else {
          left = cent + 1;
        }
        return erfen_digui(arr, val, left, right);
      }

非递归方式

function erfen_feidigui(arr, val) {
        let left = 0,
          right = arr.length - 1;
        while (left <= right) {
          let cent = left + Math.floor((right - left) / 2);
          if (arr[cent] === val) {
            return `最终查找结果下标为${cent}`;
          } else if (arr[cent] > val) {
            right = cent - 1;
          } else {
            left = cent + 1;
          }
        }
        return -1;
      }

定位左边界（查找第一个元素）

function erfen_digui(arr, val, left = 0, right = arr.length - 1) {
        if (left > right) {
          return -1;
        }
        let cent = Math.floor((right + left) / 2);
        if (arr[cent] === val) {
          /****************改动点********************/
          if (arr[cent - 1] === val) {
            right = cent - 1;
          } else {
            return `最终查找结果下标为${cent}`;
          }
          /*****************************************/
        } else if (arr[cent] > val) {
          right = cent - 1;
        } else {
          left = cent + 1;
        }
        return erfen_digui(arr, val, left, right);
      }

二分查找右边界（查找最后一个元素）

function erfen_digui(arr, val, left = 0, right = arr.length - 1) {
        if (left > right) {
          return -1;
        }
        let cent = Math.floor((right + left) / 2);
        if (arr[cent] === val) {
          /****************改动点********************/
          if (arr[cent + 1] === val) {
            left = cent + 1;
          } else {
            return `最终查找结果下标为${cent}`;
          }
          /*****************************************/
        } else if (arr[cent] > val) {
          right = cent - 1;
        } else {
          left = cent + 1;
        }
        return erfen_digui(arr, val, left, right);
      }

### 二分查找需要注意左右边界的情况